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 $122.25   6.306%   $129.9591   $129.96   $129.95   $130.00  
  Purchase #2 $19.99   7.492%   $21.4877   $21.49   $21.50   $21.50  
  Purchase #3 $19.98   6.451%   $21.2689   $21.27   $21.25   $21.30  
  Purchase #4 $90.49   9.819%   $99.3752   $99.38   $99.40   $99.40  
  Purchase #5 $211.73   10.274%   $233.4831   $233.48   $233.50   $233.50  
  Purchase #6 $14.76   10.527%   $16.3138   $16.31   $16.30   $16.30  
  Purchase #7 $146.32   9.236%   $159.8341   $159.83   $159.85   $159.80  
  7 purchase totals = $625.52       $681.7219   $681.72   $681.75   $681.80