If you have P&T 90% rating should you try for 100%. I have heard that they can lower your rating when you ask for a higher rating. I asked for a higher rating and am afraid that it will reduced. How often do they check if you don't try to get a higher rating?
No, you shouldn't "try" for a higher rating. You're correct that you have to be careful what you ask for and that once you get this started, you can actually lose benefits.
If you're at 90% and you're unable to work, the TDIU benefit is the way to go https://www.vawatchdog.org/tdiu-unemployability.html
Before you do anything, have a look here https://www.vawatchdog.org/how-to-increase-an-existing-benefit--.html
Do your homework and decide how you can win before you try to increase to 100%. Good luck.