IBG I've just retired after completing 18 years and 4000+ hours flogging a C210 around the northern parts of Australia. I too used to worry about what was acceptable turbulence until I attended a Systems and Procedures course for C210's put on by the Cessna Pilots Association. The instructors were very knowledgeable and one had worked for Cessna as a test pilot during the testing of some of these aircraft. Someone asked him when was turbulence bad enough to require cruise speed reduction. He said when you're hitting your head on the roof every 30 seconds or so. He said they are very strong and we shouldn't worry about moderate turbulence.
I tend to adjust my speed to the level of comfort I need. I'm too old to be bounced around in a hot uncomfortable cockpit in order to save 5 minutes. If it's hot and you just know it's going to be rough below 8000 stay up in the cool smooth air until the time to destination in minutes equals the number of thousand feet you have to lose. Then drop the U/C. A C210 will come down nicely @ 1000 fpm and you will see a stable 140-145 K indicated without any of those scarey 160K excursions sometimes seen descending in turbulence with U/C up.
We were told that wing failure in strutless 210's invariably occurred beyond VNE. The wing fails at the point where the flaps end and the ailerons start. The outboard section bends and breaks downwards because at very high speeds this outboard section is at a negative angle of attack due to the large washout in C210 wings. All it takes is a big aileron input (such as our hero pilot would do as he came out the bottom of a cloud and found himself looking at the ground sideways) to provide enough additional downward force to fail it.
One other worry I used to have. In dry season if you take your headset off in flight you often hear dreadful noises which sound like the carry through spar is on it's last legs. It is actually the windshield which is fitted loose so it doesn't crack. When it moves about only a minute amount it makes these noises that you would never want to hear from your spar.
So to sum up: to fly smooth, safe and worry free fly high, descend slow, don't exceed VNE and don't take your head sets off.
Cheers, RA