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 $64.80   13.329%   $73.4372   $73.44   $73.45   $73.40  
  Purchase #2 $12.48   6.162%   $13.249   $13.25   $13.25   $13.20  
  Purchase #3 $85.29   7.547%   $91.7268   $91.73   $91.75   $91.70  
  Purchase #4 $133.49   13.622%   $151.674   $151.67   $151.65   $151.70  
  Purchase #5 $27.37   9.246%   $29.9006   $29.90   $29.90   $29.90  
  Purchase #6 $59.25   11.507%   $66.0679   $66.07   $66.05   $66.10  
  Purchase #7 $151.42   4.485%   $158.2112   $158.21   $158.20   $158.20  
  7 purchase totals = $534.10       $584.2667   $584.27   $584.25   $584.20