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 $19.40   9.184%   $21.1817   $21.18   $21.20   $21.20  
  Purchase #2 $37.56   6.368%   $39.9518   $39.95   $39.95   $40.00  
  Purchase #3 $229.64   13.287%   $260.1523   $260.15   $260.15   $260.20  
  Purchase #4 $5.75   9.764%   $6.3114   $6.31   $6.30   $6.30  
  Purchase #5 $50.26   9.803%   $55.187   $55.19   $55.20   $55.20  
  Purchase #6 $204.39   9.503%   $223.8132   $223.81   $223.80   $223.80  
  Purchase #7 $242.27   6.805%   $258.7565   $258.76   $258.75   $258.80  
  7 purchase totals = $789.27       $865.3539   $865.35   $865.35   $865.50