Hi,

I had a water shut-off valve leaking a bit and tightened up the bonnet packing nut.
That corrected the leak but the valve is a bit hard to turn, not too hard, just fairly
Stiff. Can these valve stems be lubricated or is it NOT a good idea to do that?

Thanks,
Mike

Clean the stems with 400 grit wet/dry sandpaper. You find silicone plumbing lubercant at the hardware store designed for this purpose. You can back off the packing nut slightly as well.
You can also replace the packing washer.

I agree, clean it and grease it with plumbers faucet grease. You should also be able to replace the packing string.