While it's not as obvious in TW2 as it is in TW1, I still think it's fairly easy to discern how to play and what choices to make, if you decide on trying to maintain the famous witcher neutrality. I would like to delve into this (again), by posing two important questions. I also want to stress that this is of course my personal take on this subject. Everyone is entitled to disagree, if they really want to.
Would a Neutral Witcher Aid a Rebellion?
The first important choices is between Iorveth and Roche; whom to follow and obtain the aid from as the pursuit for the Kingslayer continues. To me, this is very clear-cut: A 'neutral' Geralt would not betray the person who is responsible for helping him escape the prison in the first place, making it possible for him to even set out on this chase. The help he has received from very early in the story, and the amount of trust Roche has placed in him, does not warrant Geralt suddenly stabbing him in the back and joining forces with Scoia'tael outlaws. This does not mean Geralt won't hand Iorveth his sword back, when fighting ensues; after all, the White Wolf we know is honourable, and wouldn't leave him to fend of an ambush all unarmed. But when push comes to shove, and sides need to be taken for real, then I don't see a neutral Geralt turning his back on Roche - a man who has already gone to great lengths to help him - and I definitely do not see a neutral Geralt actively helping Iorveth and Saskia out in Vergen. You can also look at it more practically. If Geralt's goal is to find the Kingslayer (and later, save Triss), he would logically stand a much greater chance at achieving this goal, if he is allied with Temeria's special forces, with Roche at its head, obviously with a good amount of resources and political pull, rather than a terrorist running around forests, who is even hated or disliked by other non-humans (as seen when he reveals himself in Vergen). So either way you look at it, the most neutral and natural choice for Geralt, is to stick with Roche for Act I and Act II. The only really tough choice here, for me, is whether or not Geralt will allow Roche to kill Henselt or not. In a way, both outcomes could be considered neutral. Allowing Henselt to walk is a politically neutral act, as killing one of the last remaining (strong) regents would further throw the Northern Kingdoms into chaos, and as much as possible, a neutral witcher would do their best to avoid this. On the other hand, if you play a neutral Geralt who is still personally invested in matters involving friends of his, he would most likely want to exact revenge upon Henselt, both for killing everyone of the Blue Stripes, and for violating Ves. Like it or not, the members of the Blue Stripes - and Ves, especially - have inevitably become something like friends of Geralt's.
Would a Neutral Witcher Choose a New Acquaintance Over an Old Friend?
In the midst of the political intrigue and Geralt's personal hunt for the Kingslayer, in an attempt to clear his name, there is one unforeseen event that changes the playing rules completely: The abduction of Triss. If there is one thing that defines Geralt, to me at least, is that he places his friends much higher on the list, than he does any sort of political intrigue or scheme. This is especially true for his older friends, those who have been at his side for many years; Dandelion, Zoltan, Ciri, Yennefer, and Triss being the fabulous five. When Triss is abducted by Letho at the end of Act I, a neutral Geralt would make it top priority to recover the stolen sorceress. Of course, this goal does not necessarily interfere with the possibility to stir up some political mischief, which is why a neutral Geralt could still help out to the best of his abilities in Vergen, if only this did not conflict with the former scenario, as he would have to betray Roche, first. Where the question posed here becomes truly prevalent, is when Triss is later captured and allegedly tortured and interrogated, by the Nilfgaardians. This is what begins the final act of TW2, and it is here the most true neutral choice appears: Would a neutral Geralt go help Iorveth free Saskia or help Roche rescue Anais, or would he turn a blind eye to either of these pleas, and simply rush to liberate his close friend Triss from the Nilfgaardian camp, where she is being tortured in who knows what gruesome ways? Again, the choice feels rather obvious to me. Even if Geralt may have formed some sort of bond with either Iorveth or Roche, and even if he might sympathise heavily with poor Anais being held captive by the despicable Dethmold; I do not see Geralt choosing either above Triss - lover or no.
So, there it is. I keep both my TW1 and TW2 playthroughs along these lines, if I want to consider them 'canon,' but I don't play like this exclusively. It's the amazing thing about these games; you are free to experiment and try out alternative approaches and make different decisions, and each time it will make the respective playthrough feel special or unique in some way or another. At the end of the day, I know which is my 'true witcher' save file, however. The struggle I mentioned above, in regards to Henselt, I usually end up killing the bastard. The neutrality may suffer slightly, but what I feel is even more important when playing both the Witcher games (and other potent character-drive games such as Mass Effect 2), is the personal investment in the story and the characters. If someone coldly executed a whole bunch of people I considered friends of mine (to some degree, at least), and brutally raped another, that person is going to die, even if he happens to be a powerful king. That's my take on it, though, and I feel I have rambled on long enough, so I'll just leave it at that.