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 $27.26   13.211%   $30.8613   $30.86   $30.85   $30.90  
  Purchase #2 $171.28   5.594%   $180.8614   $180.86   $180.85   $180.90  
  Purchase #3 $145.84   10.120%   $160.599   $160.60   $160.60   $160.60  
  Purchase #4 $67.90   12.610%   $76.4622   $76.46   $76.45   $76.50  
  Purchase #5 $111.33   3.454%   $115.1753   $115.18   $115.20   $115.20  
  Purchase #6 $47.28   5.833%   $50.0378   $50.04   $50.05   $50.00  
  Purchase #7 $46.88   10.345%   $51.7297   $51.73   $51.75   $51.70  
  7 purchase totals = $617.77       $665.7267   $665.73   $665.75   $665.80