From what I have tried, not tamping will be pretty disastrous for a machine that doesnt have any sort of preinfusion. However, machines with an e61 group or a saturated group with a gicleur seem to do OK. Not tamping certainly dirties the shower screen, and it requires a different dose, and I doubt that the results will be as consistent, but you can actually get a pretty good shot. Which sort of shoots to pieces a lot of the arguments about how crucial many features of tampers and elements of the tamping routine are.
I think that tamping is certainly necessary to maintain consistency. As long as your tamp is consistent, I think that it should be fine. Dose is, IMHO, the more important and trickier thing.