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 $234.64   11.521%   $261.6729   $261.67   $261.65   $261.70  
  Purchase #2 $38.95   4.157%   $40.5692   $40.57   $40.55   $40.60  
  Purchase #3 $90.36   9.475%   $98.9216   $98.92   $98.90   $98.90  
  Purchase #4 $113.65   13.681%   $129.1985   $129.20   $129.20   $129.20  
  Purchase #5 $95.62   6.136%   $101.4872   $101.49   $101.50   $101.50  
  Purchase #6 $247.29   6.455%   $263.2526   $263.25   $263.25   $263.30  
  Purchase #7 $135.96   11.282%   $151.299   $151.30   $151.30   $151.30  
  7 purchase totals = $956.47       $1,046.4010   $1,046.40   $1,046.35   $1,046.50