Do you go a little bit longer or shorter than the calculated length – i.e calculated spoke length is 295 and available length for DT Swiss Comp is 294 or 296. A little shorter?

Personally, I’d go shorter, any thread protruding through the nipple serves no useful purpose.

and you risk bottoming out the thread if you make a small error and end up a bit long.

Depends on the spokes/nipples as much as anything, but you can get comps in 295s so that would be my choice.