Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $60.79   8.746%   $66.1067   $66.11   $66.10   $66.10  
  Purchase #2 $86.84   8.264%   $94.0165   $94.02   $94.00   $94.00  
  Purchase #3 $72.68   4.150%   $75.6962   $75.70   $75.70   $75.70  
  Purchase #4 $209.57   12.476%   $235.716   $235.72   $235.70   $235.70  
  Purchase #5 $126.39   9.159%   $137.9661   $137.97   $137.95   $138.00  
  Purchase #6 $170.51   3.626%   $176.6927   $176.69   $176.70   $176.70  
  Purchase #7 $30.96   4.111%   $32.2328   $32.23   $32.25   $32.20  
  7 purchase totals = $757.74       $818.4270   $818.44   $818.40   $818.40