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 $222.82   12.112%   $249.808   $249.81   $249.80   $249.80  
  Purchase #2 $14.94   4.375%   $15.5936   $15.59   $15.60   $15.60  
  Purchase #3 $43.81   10.824%   $48.552   $48.55   $48.55   $48.60  
  Purchase #4 $23.80   10.265%   $26.2431   $26.24   $26.25   $26.20  
  Purchase #5 $108.98   9.477%   $119.308   $119.31   $119.30   $119.30  
  Purchase #6 $194.56   11.186%   $216.3235   $216.32   $216.30   $216.30  
  Purchase #7 $103.63   10.712%   $114.7308   $114.73   $114.75   $114.70  
  7 purchase totals = $712.54       $790.5590   $790.55   $790.55   $790.50