It's always a good idea to double check the measurements of the rim and hub, and since this is your first wheelset I'd definitely do that. The spoke calculator should always spit out the correct spoke lengths assuming the hub/rim measurements are correct. But only you can know that once you measure the parts yourself.

+1 on rounding up to the next length. On a recent build I think the calculator gave me something like 294.3 and I went with 294 (shop only stocks even spokes.) It wasn't a problem in this case but any shorter and it would've been. I'm sure I could've gotten away with 296 instead.