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 $248.94   13.263%   $281.9569   $281.96   $281.95   $282.00  
  Purchase #2 $148.38   9.662%   $162.7165   $162.72   $162.70   $162.70  
  Purchase #3 $206.95   4.265%   $215.7764   $215.78   $215.80   $215.80  
  Purchase #4 $234.75   9.595%   $257.2743   $257.27   $257.25   $257.30  
  Purchase #5 $187.69   12.495%   $211.1419   $211.14   $211.15   $211.10  
  Purchase #6 $140.75   7.738%   $151.6412   $151.64   $151.65   $151.60  
  Purchase #7 $9.93   5.552%   $10.4813   $10.48   $10.50   $10.50  
  7 purchase totals = $1,177.39       $1,290.9885   $1,290.99   $1,291.00   $1,291.00