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 $19.81   6.385%   $21.0749   $21.07   $21.05   $21.10  
  Purchase #2 $92.43   9.620%   $101.3218   $101.32   $101.30   $101.30  
  Purchase #3 $161.89   10.229%   $178.4497   $178.45   $178.45   $178.40  
  Purchase #4 $102.70   9.846%   $112.8118   $112.81   $112.80   $112.80  
  Purchase #5 $8.33   3.366%   $8.6104   $8.61   $8.60   $8.60  
  Purchase #6 $162.38   8.810%   $176.6857   $176.69   $176.70   $176.70  
  Purchase #7 $117.69   3.536%   $121.8515   $121.85   $121.85   $121.90  
  7 purchase totals = $665.23       $720.8058   $720.80   $720.75   $720.80