I don't think you need to purchase in advance as far as getting a seat on the bus;
between the 272, BlaBlaBus, and Flixbus, there are lots of departures and seats.
BlaBla and Flixbus will go up in price as the date gets closer, the 272 will not. But
not by all that much, unless you are going at a very busy time.
Geneva Airport is pretty compact, and the bus station ~100 yards from the exit
from baggage claim. So the major question is how long it takes to get bags and
clear immigration. But since you don't really need to buy a ticket ahead of time,
just have the bus departure times available for when you are ready to go.
I think bus is the way to go, but, Geneva to Annecy is only 30 miles or so, so a taxi
is not necessarily out of the question if it comes to that.