Absolutely! The silicone plunger of the cobra tap will become contaminated with a variety of organisms. You do need to unscrew the unit and make sure that any accumulations are removed and the plunger is disinfected to the degree possible.
I find that the silicone will become so embedded with 'stuff' that I have to abandon the thumb tap and replace it with another. Since they are only a few dollars, its not a big deal to toss and replace it.
Do take them apart after each keg's worth and give them a good cleaning. That will extend their replacement interval.