So as the topic implies, I'd like to know...what do you think is a good time to start a new Tau army? I know it's not confirmed, but this is more or less assuming that the army will be getting an update early next year. Even if they're not next in line, Tau are practically guaranteed to be updated
sometime in the next year or so. So here's what to think about...
#1
Fluff. Of course people will disagree about fluff, but it's something to consider. It would suck to buy a bunch of models, only to have the fluff take a turn for the worse. I wouldn't want to buy 50 Fire Warriors, only to find out that they're...I dunno...part robot or something

I'm too tired to come up with something feasible, but you get the idea.
#2
Price.
AFAIK, newer armies are usually more expensive. You could probably save several hundred dollars by buying Tau models
before they're officially updated. On a related note, availability is something else to consider. People
everywhere are currently selling and trading Tau armies. If a new update comes around, it might be near impossible to find a good deal on an army.
#3
New models. Tau probably aren't going to get a full new range of models, although they'll definitely get a few. I would feel terrible buying 10+ Crisis Suits (for example), only to have a new, better looking model released shortly after.
#4
New rules making old models "bad". There's not a lot of things in the Tau codex that can get much "worse"...but certain models could just become outclassed. How would you feel buying a ton of Broadside suits, only to have them nerfed in an update? What if Fire Warriors become absurdly good, making all of your Kroot worthless?
#5
This is probably less of an issue, considering how close the release dates might be...but 6th edition. 6th Edition could completely change the way to you want to build a list or play an army. Again, it would suck to spend tons of money and time building a mechanized Tau list (for example), only to have vehicles take a huge hit in the new edition.
I hope this doesn't come across as whining because hell, nothing has even happened yet. These are just some things to be wary of. So what does everyone think?
Would it be a good idea to start the army as soon as possible? Get a good deal, enjoy the army for the months in between now and the update, etc.
Would it be smarter to wait until after the Tau update? Possibly 5-6 month from now, but you would know exactly what codex you're working with.
Would it be best to wait until after 6th Edition? Maybe an 8-10 month wait (who knows), but there's no "risk" of making a poor decision due to rules changes.