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 $21.98   5.495%   $23.1878   $23.19   $23.20   $23.20  
  Purchase #2 $8.94   5.767%   $9.4556   $9.46   $9.45   $9.50  
  Purchase #3 $236.86   3.178%   $244.3874   $244.39   $244.40   $244.40  
  Purchase #4 $135.25   10.298%   $149.178   $149.18   $149.20   $149.20  
  Purchase #5 $28.70   12.692%   $32.3426   $32.34   $32.35   $32.30  
  Purchase #6 $77.91   11.384%   $86.7793   $86.78   $86.80   $86.80  
  Purchase #7 $104.81   9.730%   $115.008   $115.01   $115.00   $115.00  
  7 purchase totals = $614.45       $660.3387   $660.35   $660.40   $660.40