We do a lot of secondhand and clearance shopping. So because of that, we do buy things for baby before baby comes.
With Finley (who we lost at 16 weeks) we had bought a few things, and due to the loss experience, I've found that I'm okay using some of the things we bought (blankets and stuff) but not others (a certain outfit and pair of pants, but Finley was due in the winter and this baby is due in the summer, so the outfit and pants likely wouldn't get used this time anyway).
We did buy 2 Halloween outfits for this baby, right after Halloween, and paid less than a dollar for each (the onesie was 30 cents, I figure if we lose this baby, we're not out much for that! lol).
Anyway, I think it's okay to start buying whenever you feel ready! :)