KerChunking, why do it and is it legal? 

If you come from the FM repeater world, you do a quick key up to bring up the repeater to test for signal strength and confirm that the repeater can hear you.  On DMR, KerChunking primary purpose is to turn on or off a talkgroup(s), set or change timers.  Is it legal to KerChunk without providing your call sign?  Well  that depends...

Sec. 97.119 Station identification.  (a) Each amateur station, except a space station or telecommand station, must transmit its assigned call sign on its transmitting channel at the end of  each communication, and at least every 10 minutes during a communication, for the purpose of clearly making the source of the transmissions from the station known to those receiving the transmissions.

So yes, you may KerChunk but you should keep in mind, you now have 10 minutes to identify your station.  Is that practical, likely or is this simply to be overlooked?  That is your decision.  Maybe better would be a 3 second kerchunk with the words, "WA7DMR KerChunking Wash 2 Up" or simply 2 seconds, "WA7DMR KerChunk".  Or jumping at some point to ID.  All your choice.  Net controls should likely ignore any CallWatch entry less that 1 second long as it is likely a ham trying to join the talkgroup.

Also, when you change TG's, you generally are not changing frequencies and you do not "need" to ID but as a courtesy, you should.  Some folks ID or say "over" at the end of every transmission, it is unnecessary and annoying.  This is not HF.  Please turn on your call complete tone if you want to not hear "over".