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 $25.54   9.563%   $27.9824   $27.98   $28.00   $28.00  
  Purchase #2 $42.75   4.743%   $44.7776   $44.78   $44.80   $44.80  
  Purchase #3 $170.44   4.814%   $178.645   $178.65   $178.65   $178.60  
  Purchase #4 $100.73   13.795%   $114.6257   $114.63   $114.65   $114.60  
  Purchase #5 $62.68   12.428%   $70.4699   $70.47   $70.45   $70.50  
  Purchase #6 $84.72   3.254%   $87.4768   $87.48   $87.50   $87.50  
  Purchase #7 $62.72   5.423%   $66.1213   $66.12   $66.10   $66.10  
  7 purchase totals = $549.58       $590.0987   $590.11   $590.15   $590.10