On those images where I tested one round trip between Lab and RGB no image degradation popped out at me, but that of course doesn't mean very subtle changes didn't happen.
The first trip from RGB to Lab is the one that introduces the data drop-after the first trip, additional moves to and back from RGB>Lab>RGB don't do much if anything to a file.
As for exacly what happens to an RGB image that is converted to Lab, it all depends upon the colors in the image itself. The quantization errors are subtle...and as such may not show up as a problem until steps are taken to adjust the image _AFTER_ the RGB>Lab>RGB round trip.
If you are doing the conversions in 16 bit space, the quantization errors are less likely to introduce data loss that can lead to problems. That said, like anything else in Photoshop, the less erros you introduce into an image, the better the image integrety will be maintained. Thus, converting from RGB to Lab for the purpose of allpying a luminance based curves adjustment is foolish because essentially the same adjustment can be made in RGB using the curves layer in luminosity bland mode.