I have a sub-frame that the joists aren't all the same height, it looks like some various from when they were milled. I'd like to get them as consistent as possible before laying the decking on top. I know the standard approach is to plane down the high ones (and any other high points), but I don't have a few that are too high - but rather a few that are too low.

Do I need to plane all the others down to match the few lower ones, or is there a material I can put on top of the low ones to raise their height instead?

Thanks