For each of these schools, find out what the typical score is. If you're at the average accepted score or higher, then submit it. If not, then maybe it's better to omit it. Remember that 770 may not mean you are in the top X percent of all test takers, but you are not competing with all test takers, only those who are applying to your program. In my field, many departments are making the PGRE optional, and 770 would be well above the median PGRE score of an admitted student in many cases.
The other thing to check, if it's possible, is what the "optional" really means. Some programs that take in students from a large number of backgrounds (e.g. a biophysics program may accept biology and physics undergrads) might say that the PGRE is optional. But if you have a physics undergrad degree, they might expect you to have taken the physics GRE and they may assume you did really poorly if you don't submit anything. I don't know how true this is at most schools though. And it's not usually something that is easy to find out. However, if you somehow magically know that a score is expected from you based on your degree, then I wouldn't feel bad submitting an average score.