Well, in some cases it's OK, but for a quality build, I'd just get the right size, rounded down, for a new build. I round down because I hate to run out of threads. It's also best to actually measure your ERD on the rim since the published specs may not match the rim you actually buy.

That being said, I have built wheels with Shimano front and rear hubs using the same length spoke for front and rear- in my case the front hub laced 2-cross took a 295 mm spoke, the drive side rear 3X was 294 and the non-drive side rear was 296. I bought 295 for all, and mashed the threading on the drive side rear (not generally recommended). The wheel is fine, but it was for a beater wheel I don't expect much from.