I think it depends on what else you have on your bike. Like RonH and some others have said, you might see a spike in your speed sometimes just because of traffic sensors or a radar trap or something like that, and that's the only problem you'll have.

In my case, with my headlight's controller circuit, any setting other than full brightness caused enough interference that my Cateye wouldn't give a reading at all. The light's mid- and low-power "steady" settings were really just super-fast blinking, so it must have been producing a high-frequency RF buzz that freaked out the cyclometer. I didn't know this would happen, of course, so rather than ditch the light, I got a Blackburn 2.4 GHz computer instead (although I probably would've been happier with a Cateye V3 anyway).