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 $133.68   4.619%   $139.8547   $139.85   $139.85   $139.90  
  Purchase #2 $77.30   12.858%   $87.2392   $87.24   $87.25   $87.20  
  Purchase #3 $97.69   6.654%   $104.1903   $104.19   $104.20   $104.20  
  Purchase #4 $34.85   6.451%   $37.0982   $37.10   $37.10   $37.10  
  Purchase #5 $43.94   10.441%   $48.5278   $48.53   $48.55   $48.50  
  Purchase #6 $21.40   9.768%   $23.4904   $23.49   $23.50   $23.50  
  Purchase #7 $236.25   12.606%   $266.0317   $266.03   $266.05   $266.00  
  7 purchase totals = $645.11       $706.4323   $706.43   $706.50   $706.40