For what it's worth, I was on progesterone supplements for months and months, prescribed, and I hadn't had my levels checked. My doctor prescribed them based on my symptoms which were spotting from 5dpo (LP is 12-13 days).
I had to take progesterone tablets every day from cd14 for 2 weeks and then stop them. Stopping the tablets means that your normal hormones kick in and af arrives if your not pg. If you are pg, then the idea is your levels will be slightly elevated and the pregnancy more chance to survive. With my 1st child i had to take prog for the whole first trimester.
In the end, I felt in myself that the progesterone tabs weren't doing anything - sometimes I spotted, sometimes not. So we stopped them and I had my levels tested - they were normal.

But I got my bfp that month anyway, so they may be been higher due to that - it's all a mystery!
Anyway, what I'm saying is that I don't think taking progesterone can do you any harm at all, as long as you take it properly i.e. stop after 14 days and don't take it before ov. Does that make sense? SOrry feel like I'm rambling a bit!