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 $68.84   13.656%   $78.2408   $78.24   $78.25   $78.20  
  Purchase #2 $108.77   4.696%   $113.8778   $113.88   $113.90   $113.90  
  Purchase #3 $25.64   13.211%   $29.0273   $29.03   $29.05   $29.00  
  Purchase #4 $72.93   6.309%   $77.5312   $77.53   $77.55   $77.50  
  Purchase #5 $250.98   8.112%   $271.3395   $271.34   $271.35   $271.30  
  Purchase #6 $9.63   3.217%   $9.9398   $9.94   $9.95   $9.90  
  Purchase #7 $213.78   10.827%   $236.926   $236.93   $236.95   $236.90  
  7 purchase totals = $750.57       $816.8824   $816.89   $817.00   $816.70