It does not matter how good of a chef, cook or grill master you are. You should own at-least a barebones simple meat thermometer. It will save your life.
To be frank... I always guesstimated whether my meat was done, and if I had to.. I would cut into it to see. It work I was actually pretty good at telling when my steaks were medium or medium-rare (which is how I like it). But I also didn't take a lot of risks and tried a lot of different cuts.
A good friend, a very well known chef, told me I should stop wasting good meat on stupid pride and keep a good thermometer for when I wasn't 100% sure. So I invested in one and... yep, it was worth it.
Meat Thermometers are cheap... you can shell out $100's for a wireless thermometer that connects to your phone and has all kinds of nifty features... but you can also spend $9 on a simple probe and get an instant read.
I like the inexpensive thermometers because I tend to lose them...