Always add a disclaimer "oh yeah, this works only in v X.Y or newer"
Unless the version is already specified, doing this causes no harm.
Assume at least 2.6 (no one in their right mind can be running a version older than that, can they?)
Um, yes (and this one).
Assume at least 2.7 (no one should be running older than that, am I right?)
Yes, this this one to.
Go straight for 3.x (that's how Guido would like it, right)
You cannot assume that anyone's running version 3.x. It's simply not going to be true. If you're going to assume something assume someone's running 2.7 and then use the fact that there are a lot of features that are compatible between 2.7 and 3.x.
My company runs every version between 2.4 and 2.7; no one has the time or the capacity to re-write tens of thousands of lines of code that have worked for years. There's no need. There'll be plenty of others who've done the same, but where some new requirement suddenly causes them a problem. Consider try ... except ... as error in Python 2.5 - Python 3.x, for example.
If what you would answer is version dependent, you essentially have three options. In order of descending usefulness:
Ask. Comment saying "There are certain features that could be used in version 3 to solve your problem. Which version are you using?". I understand that in the cut-throat world of python answering this isn't always an option (Here's looking at you Martijn).
Write up an answer for 2.7 and 3.x These are the most likely versions that the OP will be using anyway. Be sure to label them and make the 2.7 answer 3.x compatible so that you may only need to write it once.
Your point 1. Add your disclaimer and be wary of downvotes when someone else does point 1 or 2.