I really don't think you need to apply to all of the top 20. In fact, I think you could apply to just the T14 and still be safe, especially with URM status.I can see retaking if you feel you have a good shot at the 170s and want to go to the highest ranked school possible, but if you would be happy at Penn, Michigan, Virginia, Duke, etc., I really don't think you'll have a problem, especially if you apply early.
Dude there are URMs with much lower #s who have been accepted into alot of the T14. Some in the low 160s with a 3.5ish gpa have been accepted at schools like Michigan,Georgetown,Fordham etc. With better numbers you should have a really good shot.Personally I wouldn't retake but just because I wouldn't want to deal with the stress of taking the test again. Although taking the test again and getting a 170+ could potentially get you into one of the T3. Good luck!