1mm isn't that much, so yes you can use the same for both. Which is best would depending on the direction of rounding.

Here's what to consider.

If, for example, the 290, was really 290.4 rounded down, and the 291 was really 290.6 rounded up, then both wheels really take the same spoke (difference of 0.2mm) and either is fine.

OTOH - if the 290 was really 289.6 rounded up, and the 291 was really 291.4 rounded down, then there's a difference of almost 2mm, and using either length for both will seriously reduce the working margin of error.

Also, you didn't say what kind of wheel, but most rear wheels use 2 lengths, typically 2mm apart, with the long spokes being very close to what's used for the front, and the shorter (drive side) spokes being shorter by 2mm.

So the answer to your original question is that it's impossible to give you any assurnace, and you need to check the numbers, consider the factors I mentioned and make an educated decision.