When I carried a "normal" M4, I would have one.
I would typically put the taclight's pressure pad (issued surefire) inbetween rails on the left side of the rifle, use high temp zipties to hold it on. That precluded the whole visible light AD thing to a good extent. If I had one, I'd put a scope cover on it as well to prevent it more... 9 times out of 10 we weren't visible light, so I had other things to take care of that..
I'd use the button pressure switch vs the pad, thinned and rounded with a dremel, then bike innertube to hold it in place on the front grip so I could "pull the trigger" on the front and get what I wanted.
Then, when I went to play in stryker land, the TL's carried the 203's. Yay. Back to magwell front grip with pad's epoxyed to the trigger mechanism of the 203. Taclight was rear button on the left hand side for ease of thumb actuation.
My carbine I have now sitting upstairs has a surefire TL3, mounted underneath the barrel, to where my left index finger's knuckle can actuate it. The mount is sexy though. Basically, it's a easily detatched light off a mount that stays on the rifle. Good for the "where the heck did that go" when I don't have a light easily accessable.
It's mounted pretty far back, because that's where I prefer to keep my front grip, and allows me to still assume a "normal" prone position if I want.
I *do* wish that I had been able to get a front grip like I have on my carbine, back when I was still in. Just being able to keep spare batteries in it, plus the option of mounting pressure pad's in the sides would have been something I would have appreciated.
Pic here