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 $110.31   8.777%   $119.9919   $119.99   $120.00   $120.00  
  Purchase #2 $112.78   8.333%   $122.178   $122.18   $122.20   $122.20  
  Purchase #3 $115.47   9.595%   $126.5493   $126.55   $126.55   $126.50  
  Purchase #4 $144.48   9.172%   $157.7317   $157.73   $157.75   $157.70  
  Purchase #5 $8.95   4.138%   $9.3204   $9.32   $9.30   $9.30  
  Purchase #6 $123.57   5.501%   $130.3676   $130.37   $130.35   $130.40  
  Purchase #7 $125.79   10.461%   $138.9489   $138.95   $138.95   $138.90  
  7 purchase totals = $741.35       $805.0878   $805.09   $805.10   $805.00