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 $249.30   8.287%   $269.9595   $269.96   $269.95   $270.00  
  Purchase #2 $234.33   9.184%   $255.8509   $255.85   $255.85   $255.90  
  Purchase #3 $107.79   11.275%   $119.9433   $119.94   $119.95   $119.90  
  Purchase #4 $86.49   11.155%   $96.138   $96.14   $96.15   $96.10  
  Purchase #5 $51.86   4.665%   $54.2793   $54.28   $54.30   $54.30  
  Purchase #6 $195.42   5.577%   $206.3186   $206.32   $206.30   $206.30  
  Purchase #7 $110.41   7.699%   $118.9105   $118.91   $118.90   $118.90  
  7 purchase totals = $1,035.60       $1,121.4001   $1,121.40   $1,121.40   $1,121.40