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 $100.30   12.350%   $112.6871   $112.69   $112.70   $112.70  
  Purchase #2 $78.95   10.777%   $87.4584   $87.46   $87.45   $87.50  
  Purchase #3 $44.87   12.123%   $50.3096   $50.31   $50.30   $50.30  
  Purchase #4 $54.79   8.340%   $59.3595   $59.36   $59.35   $59.40  
  Purchase #5 $187.48   7.639%   $201.8016   $201.80   $201.80   $201.80  
  Purchase #6 $134.88   10.867%   $149.5374   $149.54   $149.55   $149.50  
  Purchase #7 $168.55   5.730%   $178.2079   $178.21   $178.20   $178.20  
  7 purchase totals = $769.82       $839.3615   $839.37   $839.35   $839.40