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 $18.60   10.553%   $20.5629   $20.56   $20.55   $20.60  
  Purchase #2 $21.60   12.307%   $24.2583   $24.26   $24.25   $24.30  
  Purchase #3 $229.87   11.353%   $255.9671   $255.97   $255.95   $256.00  
  Purchase #4 $234.60   11.227%   $260.9385   $260.94   $260.95   $260.90  
  Purchase #5 $28.72   12.642%   $32.3508   $32.35   $32.35   $32.40  
  Purchase #6 $30.67   9.748%   $33.6597   $33.66   $33.65   $33.70  
  Purchase #7 $180.72   7.533%   $194.3336   $194.33   $194.35   $194.30  
  7 purchase totals = $744.78       $822.0709   $822.07   $822.05   $822.20