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 $120.71   6.491%   $128.5453   $128.55   $128.55   $128.50  
  Purchase #2 $53.38   13.333%   $60.4972   $60.50   $60.50   $60.50  
  Purchase #3 $181.54   3.733%   $188.3169   $188.32   $188.30   $188.30  
  Purchase #4 $183.83   7.257%   $197.1705   $197.17   $197.15   $197.20  
  Purchase #5 $189.50   12.248%   $212.71   $212.71   $212.70   $212.70  
  Purchase #6 $107.82   4.745%   $112.9361   $112.94   $112.95   $112.90  
  Purchase #7 $19.51   4.775%   $20.4416   $20.44   $20.45   $20.40  
  7 purchase totals = $856.29       $920.6176   $920.63   $920.60   $920.50