Wheelpro spoke calculator aims to give longest ideal length IIRC ( don't know about DT Swiss or the others) so in an ideal world this looks like 3 lots of 260 and one of 261. However, calculator usually looks to get end 1mm from end of nipple so I agree that all are within 1mm of 261 and you should be fine. Even if you cut a few new threads in the nipples this is apparently OK. Check the dish regularly and correct towards it as you go if possible.
When I bought approx 1mm under what I calculated for reasons of spoke availability and then unfortunately discovered that I had probably measured the rim 1mm or so under as well and finished up with some thread showing all round, I switched to 14mm nipples instead of 12mm to get my extra 1mm back, at which point the build went together fine, so there are ways around the problem if you were to finish up with threads showing and were concerned.