As far as I understand, lens centration is more important than end thickness, for image quality.

If the thickness is not the same after cementing, the end effect is to change the focal distance.

If you change it a lot, you may have to reset the lens flange focal distance, which typically involves a shim.

For an SLR, it may not make any difference, depending on where the new focal point ends up. e.g. if it can't focus to infinity any more, you'll likely need to move the entire lens head closer to the film, which may translate to a thinner shim.

I don't recall the tolerances, but I would look at the lens formula 1/f = 1/d1 + 1/d2 as a guide