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 $18.74   9.666%   $20.5514   $20.55   $20.55   $20.60  
  Purchase #2 $150.26   7.425%   $161.4168   $161.42   $161.40   $161.40  
  Purchase #3 $91.95   13.180%   $104.069   $104.07   $104.05   $104.10  
  Purchase #4 $39.53   12.550%   $44.491   $44.49   $44.50   $44.50  
  Purchase #5 $89.31   13.527%   $101.391   $101.39   $101.40   $101.40  
  Purchase #6 $122.99   7.709%   $132.4713   $132.47   $132.45   $132.50  
  Purchase #7 $206.89   10.326%   $228.2535   $228.25   $228.25   $228.30  
  7 purchase totals = $719.67       $792.6440   $792.64   $792.60   $792.80