It works out very well, for some things anyway. I'd say, depending on how your babies run about now -- and how "long" summer is for you (I'm in SC and we wear summer clothes from April clear through October). If your summer-clothes-wearing months are many, you'll be able to get some 18month sized clothes and what doesn't fit at the beginning of the summer should at least come close by the middle-end.
I wouldn't go ALL OUT at this stage, but if you find some bargains, by all means go for it. Some things work out better for this than others. My kids are both on the skinny side. I don't normally buy pants for them a year out (unless it's an incredible bargain and I find it worth the risk) because by the time they fit in the waist, they might be out of season or too short. But shirts and dresses, I have NO problem buying those ahead.
Good luck - I also love re-sale shopping. I LOVE bargain hunting. Goodwill is my friend!
Tracie