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 $19.76   10.330%   $21.8012   $21.80   $21.80   $21.80  
  Purchase #2 $153.62   6.874%   $164.1798   $164.18   $164.20   $164.20  
  Purchase #3 $154.63   3.228%   $159.6215   $159.62   $159.60   $159.60  
  Purchase #4 $170.77   10.258%   $188.2876   $188.29   $188.30   $188.30  
  Purchase #5 $34.62   9.550%   $37.9262   $37.93   $37.95   $37.90  
  Purchase #6 $40.63   13.707%   $46.1992   $46.20   $46.20   $46.20  
  Purchase #7 $38.31   7.306%   $41.1089   $41.11   $41.10   $41.10  
  7 purchase totals = $612.34       $659.1244   $659.13   $659.15   $659.10