I thought Coinomi was a very trusted piece of software. Why send the BTC to another address before using Coinomi if Coinomi is trustworthy? I don't understand why you would allow an application to see your private key for BCH yet not for BTC. I know Bitcoin is worth a lot more, but there is tremendous incentive to steal BCH as well.
Are they trustworthy? I guess. But everyone is trustworthy until they aren't. Right?
Not sure why you wouldn't send the BTC to another address though. It's just an extra precaution that doesn't really cost you anything (well except for the transaction fee of course). Why take an unnecessary risk?
Also, you ARE letting the application see your private key for BTC because the BCC and the BTC share the same address. That is why you're moving your BTC in the first place.