Sorry if this is a lame-noob question.

I want to get DT Comps. I used DT's spoke calculator and it recommended 291 for the front wheel and 290 for the back using 12mm nipples. Can I just get all the spokes one size? Which of the 2?

Thanks!

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.

Is there an inherent reason you want all the same size?
I know if you want to buy a box of 72, you may save money.
I pay a bit more and get my spokes here-
DT Swiss Spokes

You can buy them by the spoke.
I like this because I can use a different gauge spoke on either side of the rear.
On my hybrid 32 spoke wheel, I use 14-15 DB on the rear DS and 15-16 everywhere else.

It's a track-wheel setup, so same size spokes on each side of a hub.

I was just wondering since, yeah, I see boxes of 72 spokes being sold everywhere and wondered if 1 mm would even make a difference. It probably depends on rounding like the FBinNY said.

Bill, IMO the 1.8/1.6/1.8 spokes are not really a good choice. Since spoke breakage nearly always occurs at the ends, it is unwise to take metal off of those places. IMO you would be better served to use the even lighter Revolutions, 2.0/1.5/2.0. Yes, the centers are thinner, but that is not a problem, because there is plenty of strength there for the applied stresses in that area. The important thing is that you have 2.0 mm on both ends where you need it. A second choice would be Super Comps, 2.0/1.7/1.8 with a substantial J-bend gauge as well as a little more metal in the centers. At least you get the extra metal on the highly stressed J-bend if not at the threaded end, and this would still be better than the 1.8/1.6/1.8 variety. Also the Super Comps have shorter butted ends, so they actually weigh almost the same (only 7 g difference) as the 1.8/1.6/1.8 Comps. Just trying to help!

After dropping my rear wheel (Sun Rims M13 II w/28mm tire) into an old fashioned sewer grate and bouncing my 240 lb. butt 8-10" out of the seat, I'll stand by my recipe.
It knocked the wheel about 2mm out of true. 20 minutes with the truing stand and tension meter and I had the wheel true and tensions back within 3%.
One might make a case that the 1.8mm J bend is already "pulled" further and thus has less potential distance to move/bend.

Well, of course, what works for you is what is best. I would only point out that spokes seldom fail due to acute stresses but rather due to fatigue from chronic, repetitive stresses over long periods of time. So the fact they withstood an abuse like say a pot hole is not really germane to the issue. Case in point: The popular Revolution spokes I mentioned above used to be 1.8/1.5/1.8. When DT wanted to make them more durable, they did not make the very fine 1.5 section heavier. They made the ends 1.4. Just trying to help.

Yeah, I checked the calculator, and the front is 290.7 and back is 290.4. It rounded up and down respectively. Seems like a really small difference.