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 $19.47   8.599%   $21.1442   $21.14   $21.15   $21.10  
  Purchase #2 $8.43   7.150%   $9.0327   $9.03   $9.05   $9.00  
  Purchase #3 $169.67   11.307%   $188.8546   $188.85   $188.85   $188.90  
  Purchase #4 $146.42   7.387%   $157.236   $157.24   $157.25   $157.20  
  Purchase #5 $190.42   9.506%   $208.5213   $208.52   $208.50   $208.50  
  Purchase #6 $81.54   13.446%   $92.5039   $92.50   $92.50   $92.50  
  Purchase #7 $156.84   3.567%   $162.4345   $162.43   $162.45   $162.40  
  7 purchase totals = $772.79       $839.7272   $839.71   $839.75   $839.60