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 $40.84   11.703%   $45.6195   $45.62   $45.60   $45.60  
  Purchase #2 $78.30   7.739%   $84.3596   $84.36   $84.35   $84.40  
  Purchase #3 $221.62   12.503%   $249.3291   $249.33   $249.35   $249.30  
  Purchase #4 $149.30   12.600%   $168.1118   $168.11   $168.10   $168.10  
  Purchase #5 $26.98   4.547%   $28.2068   $28.21   $28.20   $28.20  
  Purchase #6 $19.96   12.836%   $22.5221   $22.52   $22.50   $22.50  
  Purchase #7 $37.77   7.300%   $40.5272   $40.53   $40.55   $40.50  
  7 purchase totals = $574.77       $638.6761   $638.68   $638.65   $638.60