I've gotten cocky before and eyeballed smoke lengths- I had a stock of spokes with maybe 20mm of threads for a while. Usually it worked out, but when it didn't it was a pain in the backside to file/dremel down the nub of spoke coming through the nipple.

It's not a deal breaker if you have enough thread, but if you have single wall rims or grossly miscalculated, be ready to file/cut.