I have a Reiff preheat system on my 170. Here's their FAQ page, related to leaving the heat on continuously (it's OK to do that): http://www.reiffpreheat.com/FAQ.htm#QA3
. It refers to an Aviation Consumer article that might have more to say -- I haven't read it.
I used to be a member of a flying club here that had five Lycoming-powered Cessnas with Tanis heaters, and they plugged them in whenever they were in the hangar. They've done this literally for decades with no ill effects.
When I had my 170 in a cold hangar, I put the Reiff system on a timer to warm it up for the weekend (I was working then, and did all my flying on weekends. I didn't fly every weekend, so sometimes I was heating the engine for nothing (i.e. cycling the heat). I started to see rust in my oil filter, so I stopped doing that. When I bought the Reiff system, they recommended turning it on twelve hours before flight, then leaving it off until the next flight.
I'm now in the habit of removing the oil filler cap after each flight, so that condensation can escape. As long as you're not worried about bugs and things getting in there, I think that helps keep the engine clean. I don't know for sure, but it might have helped when I was cycling the heat -- but I wasn't doing it then.
Now I'm in an insulated hangar with radiant heat. In cold weather I keep the thermostat set at 46 degrees when I'm not there, and I don't preheat any more unless I RON somewhere. That's about the best compromise I can think of.
So my recommendation to you is either to leave heat on the engine full-time (maybe just a light bulb inside the cowling, in your mild climate) or warm it only before a flight.