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 $15.92   13.262%   $18.0313   $18.03   $18.05   $18.00  
  Purchase #2 $17.27   11.507%   $19.2573   $19.26   $19.25   $19.30  
  Purchase #3 $114.90   3.394%   $118.7997   $118.80   $118.80   $118.80  
  Purchase #4 $115.72   5.646%   $122.2536   $122.25   $122.25   $122.30  
  Purchase #5 $65.36   4.618%   $68.3783   $68.38   $68.40   $68.40  
  Purchase #6 $30.71   9.701%   $33.6892   $33.69   $33.70   $33.70  
  Purchase #7 $7.61   3.785%   $7.898   $7.90   $7.90   $7.90  
  7 purchase totals = $367.49       $388.3074   $388.31   $388.35   $388.40