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 $19.80   9.793%   $21.739   $21.74   $21.75   $21.70  
  Purchase #2 $82.63   8.709%   $89.8262   $89.83   $89.85   $89.80  
  Purchase #3 $98.70   3.585%   $102.2384   $102.24   $102.25   $102.20  
  Purchase #4 $114.42   12.839%   $129.1104   $129.11   $129.10   $129.10  
  Purchase #5 $51.91   9.696%   $56.9432   $56.94   $56.95   $56.90  
  Purchase #6 $189.50   3.343%   $195.835   $195.84   $195.85   $195.80  
  Purchase #7 $59.96   5.430%   $63.2158   $63.22   $63.20   $63.20  
  7 purchase totals = $616.92       $658.9080   $658.92   $658.95   $658.70