Also be aware that 2x3x4mm LED's installed on "correct" orientation switches may need to be shaved down so as not to interfere with the keycaps.
As moose said, it depends how bright the LEDs are and the construction of the caps. most backlits will allow at least some light through, but you will also get almost as bright light shining out from under the cap. I have "correct" orientation switches in my KBT Pure with round LED's in and some caps don't look good with the backlighting on. I actually prefer thick, dark opaque caps that only allow the background glow effect with that board.