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 $27.78   3.280%   $28.6912   $28.69   $28.70   $28.70  
  Purchase #2 $130.57   11.770%   $145.9381   $145.94   $145.95   $145.90  
  Purchase #3 $98.63   6.759%   $105.2964   $105.30   $105.30   $105.30  
  Purchase #4 $21.46   9.424%   $23.4824   $23.48   $23.50   $23.50  
  Purchase #5 $66.88   13.253%   $75.7436   $75.74   $75.75   $75.70  
  Purchase #6 $250.73   11.625%   $279.8774   $279.88   $279.90   $279.90  
  Purchase #7 $214.38   3.143%   $221.118   $221.12   $221.10   $221.10  
  7 purchase totals = $810.43       $880.1471   $880.15   $880.20   $880.10