I sometimes check mags at cruise power in the air - but only over flat
country. I also occasionally check them at idle on the ground to keep
track of the condition of the points.

One caveat is if you get a rough engine that is fixed by going to one
mag, don't experiment with the BOTH position again. One fellow I knew
did that on a 182 and had an exposion in the intake manifold that blew
off some of the intake system hoses. The integrity of the intake
system is critical to maintaining a semblance of the correct mixture
and the open port(s) made things so lean he couldn't recover power.

The range of spark ignitable mixture is less than three to one air/fuel
ratio (from about 8:1 to maybe 20:1 by weight).