At least vary the distance A mat won’t teach you anything about differing turf conditions.

….If I could use a mat, I think I could drop 3-5 strokes a round. I don’t get enough practice with deep, lush rough.

It's not dangerous, but it does not give you any real indication of how well you will chip on grass. Listen to the contact. If you hear crisp ball contact, that's a plus. If you hear the club strike the mat first, you're in trouble.

Personally, as long as you're hitting ball first and accelerating through impact, you're developing good habits. The club will bounce on the mat, depending the type of surface, but nothing that is going to hurt your game. That's just MY opinion, I'm a pretty good wedge player. The difference is the feel/touch you can only acquire practicing near a chipping area/green.

I chip a lot off a mat in my backyard as well. I have a mat that has 2 different grass lengths and chip balls off both sides. I have noticed that my short game is better as a result of the consistency of my practice.

I don't think it will hurt much but as Don said, you won't get different conditions of feel but if you are just focusing on keep the short game in check, it's good.