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 $220.45   3.579%   $228.3399   $228.34   $228.35   $228.30  
  Purchase #2 $163.32   9.418%   $178.7015   $178.70   $178.70   $178.70  
  Purchase #3 $40.75   13.329%   $46.1816   $46.18   $46.20   $46.20  
  Purchase #4 $13.53   11.108%   $15.0329   $15.03   $15.05   $15.00  
  Purchase #5 $78.25   9.648%   $85.7996   $85.80   $85.80   $85.80  
  Purchase #6 $17.94   10.540%   $19.8309   $19.83   $19.85   $19.80  
  Purchase #7 $37.97   5.625%   $40.1058   $40.11   $40.10   $40.10  
  7 purchase totals = $572.21       $613.9922   $613.99   $614.05   $613.90