I personally run 18* of initial, 36* total mechanical. My vac advance can adds 16* and only needs 7 inches of vacuum to pull in fully. It is a factory points can for a hi po vette from back in the day. It works good with my setup since it is only making 14" at idle. This gives me about 52* total timing, I run pump gas and never have pinging issues even in hot weather. My personal best was 14MPG so far. Still working on it. I have it hooked up to manifold vacuum too, it really helps clean up my idle without causing starter kickback that additional initial would cause.
I have no stumbles, and my throttle response is awesome. It's perfectly safe to run high advance numbers at cruise and idle, there is little to no load on the engine and the leaner mixtures take longer to burn so they really need to be started earlier in order to maintain efficiency. All modern cars also take advantage of this too with computer controlled timing.
As soon as you crack the throttle open the vacuum advance starts dropping out, lowering to what it should under load. A properly selected mechanical advance curve and vacuum can should cause no problems on a street car.
I initially had mine set up with an advance curve that was all in before 3K and nothing else and I was not happy with it. Mileage was horrible and my idle was much choppier.
