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 $63.84   3.831%   $66.2857   $66.29   $66.30   $66.30  
  Purchase #2 $9.26   12.589%   $10.4257   $10.43   $10.45   $10.40  
  Purchase #3 $111.33   13.465%   $126.3206   $126.32   $126.30   $126.30  
  Purchase #4 $69.62   4.653%   $72.8594   $72.86   $72.85   $72.90  
  Purchase #5 $199.38   3.659%   $206.6753   $206.68   $206.70   $206.70  
  Purchase #6 $69.68   4.836%   $73.0497   $73.05   $73.05   $73.00  
  Purchase #7 $69.95   4.455%   $73.0663   $73.07   $73.05   $73.10  
  7 purchase totals = $593.06       $628.6827   $628.70   $628.70   $628.70