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 $29.93   8.559%   $32.4917   $32.49   $32.50   $32.50  
  Purchase #2 $72.68   5.496%   $76.6745   $76.67   $76.65   $76.70  
  Purchase #3 $58.35   7.518%   $62.7368   $62.74   $62.75   $62.70  
  Purchase #4 $114.89   4.515%   $120.0773   $120.08   $120.10   $120.10  
  Purchase #5 $195.40   9.327%   $213.625   $213.63   $213.65   $213.60  
  Purchase #6 $127.90   4.276%   $133.369   $133.37   $133.35   $133.40  
  Purchase #7 $109.95   13.610%   $124.9142   $124.91   $124.90   $124.90  
  7 purchase totals = $709.10       $763.8885   $763.89   $763.90   $763.90