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 $18.81   12.293%   $21.1223   $21.12   $21.10   $21.10  
  Purchase #2 $166.52   4.800%   $174.513   $174.51   $174.50   $174.50  
  Purchase #3 $250.86   4.638%   $262.4949   $262.49   $262.50   $262.50  
  Purchase #4 $100.71   13.490%   $114.2958   $114.30   $114.30   $114.30  
  Purchase #5 $188.92   11.575%   $210.7875   $210.79   $210.80   $210.80  
  Purchase #6 $215.93   9.395%   $236.2166   $236.22   $236.20   $236.20  
  Purchase #7 $204.37   3.359%   $211.2348   $211.23   $211.25   $211.20  
  7 purchase totals = $1,146.12       $1,230.6649   $1,230.66   $1,230.65   $1,230.60