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 $18.78   7.423%   $20.174   $20.17   $20.15   $20.20  
  Purchase #2 $63.46   13.782%   $72.2061   $72.21   $72.20   $72.20  
  Purchase #3 $11.95   10.410%   $13.194   $13.19   $13.20   $13.20  
  Purchase #4 $143.37   10.507%   $158.4339   $158.43   $158.45   $158.40  
  Purchase #5 $73.80   7.714%   $79.4929   $79.49   $79.50   $79.50  
  Purchase #6 $228.68   6.441%   $243.4093   $243.41   $243.40   $243.40  
  Purchase #7 $123.61   7.177%   $132.4815   $132.48   $132.50   $132.50  
  7 purchase totals = $663.65       $719.3917   $719.38   $719.40   $719.40