Some teamhits will always happen.
The game can and does lag sometimes (I get the occasional, say, one in 2-3 maps, lagspike where the computer is unresponsive for 5 seconds, if I'm on horse, well, fuck it, I'll trample someone in a random direction); people have best intentions to help a teammate sometimes, cavalrymen and archers esp., but a friendly moves in a totally unexpected direction and gets hit instead, and so on. Sometimes you try to help someone being gangbanged 5v1 and end up shooting the teammate in the head.
I would rather have horsemen help (if he does something totally atrocious/idiotic I'll press M), then allow some hero to kill half of the team. Even if it results in a friendly couch/trample/stab sometimes. Same with archers. Sure, a peasant who can't be accurate because game mechanics and stuff, I'll press M. If it's Bagge helping in a melee and I get shot, well, crap. I'll rather have Bagge shooting into melee then not shooting into melee.
If you think someone made an idiot move, you report, if it was a case of "shit happens", then no reason to report. I report maybe every 5h teamhit or so, myself, when someone was clearly making an error.
That said, it would not hurt to reduce the counter to, eg. 4 (or maybe even only 3). I would have to be on horse and very careless/drunk to get more then 3 reports. And increase the timer to 25 seconds. Since sometimes you're too busy to press M.