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 $234.75   12.682%   $264.521   $264.52   $264.50   $264.50  
  Purchase #2 $45.41   12.184%   $50.9428   $50.94   $50.95   $50.90  
  Purchase #3 $57.99   13.697%   $65.9329   $65.93   $65.95   $65.90  
  Purchase #4 $43.74   6.591%   $46.6229   $46.62   $46.60   $46.60  
  Purchase #5 $18.47   4.532%   $19.3071   $19.31   $19.30   $19.30  
  Purchase #6 $112.69   6.411%   $119.9146   $119.91   $119.90   $119.90  
  Purchase #7 $196.27   6.177%   $208.3936   $208.39   $208.40   $208.40  
  7 purchase totals = $709.32       $775.6349   $775.62   $775.60   $775.50