Make sure your valve/stem is fully open when you try to unscrew it
I don't recommend heating up the valves with any kind of torch. You could end up with an emergency call to a plumber to fix what ever might get botched to get your water back on.
The last time a DIY (weekend warrior) tackled a shower faucet with a torch, he spent 12 hours on the thing and finally gave up, called me at midnight because his tenants have had no water all day. The entire faucet fell apart before he called me