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 $239.27   12.141%   $268.3198   $268.32   $268.30   $268.30  
  Purchase #2 $218.29   13.583%   $247.9403   $247.94   $247.95   $247.90  
  Purchase #3 $122.84   8.257%   $132.9829   $132.98   $133.00   $133.00  
  Purchase #4 $104.62   8.852%   $113.881   $113.88   $113.90   $113.90  
  Purchase #5 $114.47   3.122%   $118.0438   $118.04   $118.05   $118.00  
  Purchase #6 $94.54   9.542%   $103.561   $103.56   $103.55   $103.60  
  Purchase #7 $127.27   4.586%   $133.1066   $133.11   $133.10   $133.10  
  7 purchase totals = $1,021.30       $1,117.8354   $1,117.83   $1,117.85   $1,117.80