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 $59.57   11.168%   $66.2228   $66.22   $66.20   $66.20  
  Purchase #2 $241.32   11.793%   $269.7789   $269.78   $269.80   $269.80  
  Purchase #3 $117.97   9.719%   $129.4355   $129.44   $129.45   $129.40  
  Purchase #4 $91.75   4.687%   $96.0503   $96.05   $96.05   $96.10  
  Purchase #5 $127.30   3.271%   $131.464   $131.46   $131.45   $131.50  
  Purchase #6 $25.58   4.686%   $26.7787   $26.78   $26.80   $26.80  
  Purchase #7 $52.73   10.361%   $58.1934   $58.19   $58.20   $58.20  
  7 purchase totals = $716.22       $777.9236   $777.92   $777.95   $778.00