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 $18.48   11.146%   $20.5398   $20.54   $20.55   $20.50  
  Purchase #2 $7.51   11.478%   $8.372   $8.37   $8.35   $8.40  
  Purchase #3 $46.91   6.520%   $49.9685   $49.97   $49.95   $50.00  
  Purchase #4 $37.71   6.749%   $40.255   $40.26   $40.25   $40.30  
  Purchase #5 $230.88   5.703%   $244.0471   $244.05   $244.05   $244.00  
  Purchase #6 $33.81   9.751%   $37.1068   $37.11   $37.10   $37.10  
  Purchase #7 $101.41   8.287%   $109.8138   $109.81   $109.80   $109.80  
  7 purchase totals = $476.71       $510.1030   $510.11   $510.05   $510.10