JPEG quality is a pet peeve of mine. The point is just this: don't output JPEG images at very low quality just to make smaller files. Your images will look terrible.
I suggest that a level of 7 to 8 out of 10 is fine. Very high quality doesn't look much different than level 7, and the files are much larger.
Bottom Line: Output your JPEGs at roughly quality level 7.
See how the eyes are just weird fuzz. Quality is way too low.
Still too low quality. The eyes are still fuzzy.
Quality level 6 out of 10 is almost good enough.
Quality level 7 out of 10 is fine.
Quality level 8 out of 10 is plenty.
Quality level 9 out of 10 is not necessary.
Quality level 10 out of 10 is not necessary.