What is the consensus on rounding off spoke lengths from calculators when then purchasing spokes? Longer or shorter, e.g, if I calculate 291.5, would you then buy 292 or 291?

down - always, no exception

Down! One of the worst possible things you could run into is too long of a sopke that bottoms out (runs out of thread) so you cannot reach the optimum spoke tension. Then you can't properly do the build and may end up out of true and/or worse with a non-durable, weak wheel.

again: round down and more than just the fractional difference: I've found most of the on-line calculators I've used are off by at least 1mm too long, and often by 2-3mm.

It depends what your other spokes calculate out to. Sometimes for rear wheels you can round one side up and one side down which allows you to buy 1 length spoke versus 2.

+1
-Gene-

AFAIK most spoke calculators don't compensate for spoke stretch. If you use thin spokes, e.g. DT Revolution spokes (2.0/1.5/2.0mm), the stretch can be quite significant.

The spoke calculator on DT-Swiss' site does compensate for stretch for their own types of spokes, and also for different nipple lengths (if you use DT's nipples). It also rounds off to 1mm increments:
https://www.dtswiss.com/spokescalc/we...px?language=en

It's worth trying and comparing to the results from other spoke calculators.