I'm also wondering this myself.
In the past I have taped using Mic in and have had no problems as of yet - but do you suggest I try Line In next time??
My equipment:
Sony MZ-RH910 and (soon to be) Cardioid SP-CMC-19 mics (thanks to wiggle)
If I use line in, do I need to boost the recording level up a touch??
Cheers,
Simon.