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 $239.61   6.619%   $255.4698   $255.47   $255.45   $255.50  
  Purchase #2 $197.71   13.621%   $224.6401   $224.64   $224.65   $224.60  
  Purchase #3 $62.71   6.117%   $66.546   $66.55   $66.55   $66.50  
  Purchase #4 $111.94   3.771%   $116.1613   $116.16   $116.15   $116.20  
  Purchase #5 $67.40   12.117%   $75.5669   $75.57   $75.55   $75.60  
  Purchase #6 $119.55   11.744%   $133.59   $133.59   $133.60   $133.60  
  Purchase #7 $241.67   4.141%   $251.6776   $251.68   $251.70   $251.70  
  7 purchase totals = $1,040.59       $1,123.6517   $1,123.66   $1,123.65   $1,123.70