It depends. If you want a new playtrough you could start now as i don't think there are any major bugs related to new games, only saves carried trough many patches might suffer.
However, if you want more content than at the moment i'd suggest waiting at least for the next week as there is a chance we'll get a new beta patch with many new things(don't go mad if it doesn't, i just speculated as mexxico said the changes are already in).
I think it's really hit or miss on bugs, I haven't hit a game breaking one and I've been playing since release, whereas some people are still having trouble turning the game on... overall you'd probably be pretty safe based from what I've seen lurking on the forums. However I'd wait until they get the next update out as they are planning on adding some very nice new features(though they may also cause more bugs, so...).
160ish days on 1.5.5 beta and current hot fixes and so far I can't say I've had a bug tbh. Granted I don't really do quests so that dodges a landmine of potential bugs right these. I will say I was very disturbed by the war declarations of the Northern empire after I joined them to try and see the normal intended game path TW is working on.... They after get getting tag teamed by southern empire and Khuzait, had a short peace then declare war on S.empire... okay sounds fine...1 week later declare war on Khuzaits.
Hopefully something in 1.5.6 with the snowball balance will deter this AI self sabotage. I just can't see the math behind why the chose 2 wars.
I guess they were looking at S.emoire and khuzaits each being in a war allready.... but the AI I guess can't understand that being in between both they will become the easy target.