Here's a conundrum:

I'm building a set of wheels, and I calculated my spoke length using an online spoke length calculator. I got this for my spoke length: 268.5mm.

Ok. No prob!

However, the spokes I want to use only come in even numbered increments. So, should I round down to 268mm or up to 270mm? For 14mm nipple? For 12mm nipple? My gut tells me rounding down to 268mm would be more prudent, in that by rounding up, the extra 1.5mm may protrude above the top of the nipple and possibly make tightening to the desired tension a little tough.

I'm not sure of the nipple length, off the top of my head.

Let me know what you think.

Thanks.

Did your calculator assume the bottom of the screw driver slot?
IF using a double wall rim, a bit of extra length won't hurt, as long as you don't run out of threads.
.5mm short of the screw driver slot probably will be OK, but not ideal.

Assuming your calculator is within .5mm is rather wishful thinking.
Your published ERD can also vary from reality.

I generally round up. The calculators are almost never exact but they get you close. If you are doing single wall rims and the spokes are a millimeter or two too long you can always dremel the end off.

Down.
Measure your ERD yourself.
Not all 14mm nipples have same amount of threads.
Lots of good info out there. https://www.ergottwheels.com/wheelbuilding101pti.html
Wheel Fanatyk - Wheelbuilding Lore, Remarkable Tools, and Expert Advice.

DT spokes and 12mm nipples only allow 1.5mm of thread beyond the top before they bottom.

2.0/1.8 DS and front spokes will stretch about that far.

By 2mm, which is enough to break an alloy nipple with a spoke supposed to end 0.5mm below the nipple slot or bottom one supposed to end .5mm beyond its top.

This is why measuring is essential.

But... If the spokes are too long, the end will come out the top. They won't be half seated as shown. That photo is very misleading.

All you need to do is look where the bottoms are marked equally.
Just screw the nipple on the last 2 threads and you still have the same offsets.

Too short and nipple heads tend to break off.

Do you realize how small .5mm is? That represents only about 2 spoke threads (56tpi)? Rounding down that little will cause no problems at all.

I always round down. Nothing worse than running out of threads.