The only time I have my hand on the shifter is when I am changing gears. Do what you like. The day you hit something & the steering wheel is wrenched out of your hand you will understand.
I just remember my (police pursuit) driving instructor saying "if you've got two hands, why not use both of them?". It's interesting to see in the latest WRC rally cars with manual sequential gearchanges how some drivers (notably Jari-Matti Latvala) will leave their hand on the gear lever between upchanges on full acceleration (so maybe only 3 seconds between each) - only on straights, but twitching under wheel slip even with LSDs at 100mph on gravel would need two hands if I was driving..... Serious drivers don't corner hard with one hand on the wheel if they don't have to - though again, guys like Latvala seem to be able to apply 90deg of steering lock with one hand, if they have to change gear at the same time. But I think one would have to look long and hard to find anyone taking the Corkscrew at Laguna Seca with one hand on the wheel. Maybe another difference between American and European expectations is that I drive each week on minor roads like the Corkscrew - but narrower and more poorly surfaced - whereas they wouldn't be tolerated by North American drivers?
Rally drivers are somewhat above us mere mortals, as are their cars. The times where I've had to let go of the wheel mid corner to try & grab a gear after one of my screw ups always got my attention.
I also only remove my right hand from the wheel when I am shifting, and immediately return it to the 3:00 position. Aside from having less wheel control, I don't want to be putting any inadvertent pressure against the shifter, slowly grinding up any transmission parts.
I have plenty of control with my one hand even on rough roads. Also I don't put pressure on my gear shifter but I find it less distracting to just leave my hand there than to keep moving it back and forth.
Two hands on the wheel, shift, return to wheel. Not like you have to shift 50 times in one turn. It's not fast and the furious.
Set up for the turn, before the turn. No need to have your hand on the shifter. You shouldn't be doing anything in the turn except turning and using the throttle. Jim
I usually shift down going in, then shift up coming out so it would be pointless to move my hand. In any case I have more than enough control. I've not lost the wheel on a turn yet and been driving like that for 9 years.