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 $27.42   9.680%   $30.0743   $30.07   $30.05   $30.10  
  Purchase #2 $13.79   9.527%   $15.1038   $15.10   $15.10   $15.10  
  Purchase #3 $210.82   4.500%   $220.3069   $220.31   $220.30   $220.30  
  Purchase #4 $75.64   7.428%   $81.2585   $81.26   $81.25   $81.30  
  Purchase #5 $200.87   12.452%   $225.8823   $225.88   $225.90   $225.90  
  Purchase #6 $130.71   7.338%   $140.3015   $140.30   $140.30   $140.30  
  Purchase #7 $66.87   7.514%   $71.8946   $71.89   $71.90   $71.90  
  7 purchase totals = $726.12       $784.8219   $784.81   $784.80   $784.90