Spreadsheet

TLDR: Up to 58 Tower Damage, 0 Tower Rate is most efficient per point. Past that is as follows (Tdam,Trate):

60,3 70,4 80,5 95,6 125,7 150,10 200,15 250,20 300,25 500,50 1000,100 2500,250.

Without armor you would want to wait until level 21 (63 point total) to allocate any points into tower rate. The pattern somewhat follows 20:1 before 150 total points, 15:1 before 500 total points, and 10:1 beyond that.