I would say go for it - and it sounds like you did! I'm glad, that's a really cute and versatile dress. You strike me as a very disciplined person, so your 10-pound goal sounds achievable.
I take those gambles, if I know I'm close to fitting the item - and I know I can use the item long-term (i.e., over a couple of years, through periods where I might gain and re-lose a few pounds).
I've gotten better at judging what is a good gamble - but again, I approach it as a long-term project, knowing that if there is a holiday splurge and I gain some weight back, it will take some time to lose it again.