There are ways in which a wireless can be detrimental to your tone. They do not present the same load to your pickups, and of course they do not pass the load changes to your amp the way a cable does.
To translate that; if you are a cable straight to the amp guy, it will change your tone and will drastically change the way your amp responds to your touch and you volume control. (If you have ever turned an old Deluxe or Champ up to 10, you know what I'm talking about.) This is mostly an issue if you are using a non-master volume amp, and the simpler your amp the more it will matter (it will not matter nearly so much on a Silver Face Twin as on a Tweed Twin, for instance).
If, however, you have any effects in between your guitar and your amp, then a wireless won't hurt anything, and it will act as a buffer amp for your old fashion analog effects (Fuzz Faces, Wha-whas, etc.) which load down your pickups and kill the highs. If you are using anything fairly modern (any Boss effect, for instance) that has a built in buffer, it won't matter much, although some people with seriously good ears will tell you wireless units lack dynamic range. I've never played with them enough to notice.
Oh, and to the guy who said professionals all use wireless, not even close. I work as a stage hand, and I'm up on a lot of big stages. About half the guitar players use wireless, the other half use cables. It comes down to what each guy prefers. Most blues guys use cables, Rock guys tend towards wireless (though not always) and Country guys are about 50/50. And that is just the guys doing big tours. Most proffesional musicians I know (defined in the only way that matters - they make their living playing music) would never bother with a wireless, as it makes no sense when the stage is only shorter than your 20 foot cable.
Light
"Cowards can never be moral."
M.K. Gandhi