Rising junior here. I’m a bit bored this summer and was wondering if it would be a good idea to start taking a look at some prep books since I plan on taking a few subject tests this year. (Is taking them junior year a good idea?) The specific tests I’m thinking about taking are Math Level II, Physics, and Chemistry. I took Honors Chemistry this past year (sophomore year), and will be taking Physics and Precalculus this coming year.
Definitely! I’ll be a senior next year and the one thing I regret is not taking subject tests sooner. I would say definitely take the Chemistry one at the next available testing date since you just finished the class, with some prep you could probably do well on Math II as well. For Physics I think it would be best if you waited until you completed the class to take the subject test. Good luck!
Glad to see you’re taking this seriously 
I recommend taking subject tests if you took the AP or if you’ve prepped for a good 2-3 months and have a solid backing in the skill. Taking them junior year is good, I took Chemistry as a sophomore and saw freshmen taking SAT tests that early.
Good luck!