i've been doing this....and a lot! to be fair they are items that fit me, but they aren't clothes that i would grab to wear. for example, a jacket that fits, but doesn't exactly button without a bit of bunching or whatever. i'm currently in a size large, but sometimes the larges are too big and the mediums are a bit small so i just buy the medium and in another 5-15 lbs i think it would look really great.
clothes are expensive and i feel like even if i lost 25-30lbs these pieces i've been getting will still look fantastic on my smaller body and fit pretty nicely. I/E silky tunic top, camisole, shrugs, form fitting light jackets that will just look a bit looser, but not frumpy. go for it, just don't go overboard and be too daring about it. get a few things in one size down and have fun with it. clothes are expensive-i buy things here and there now so i can have a nice stockpile when i reach that size. i know it won't be too far out, just a few months for myself. i also do makeups here and there and shoes/accessories.