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 $53.72   5.699%   $56.7815   $56.78   $56.80   $56.80  
  Purchase #2 $43.93   10.176%   $48.4003   $48.40   $48.40   $48.40  
  Purchase #3 $221.93   11.747%   $248.0001   $248.00   $248.00   $248.00  
  Purchase #4 $81.53   12.772%   $91.943   $91.94   $91.95   $91.90  
  Purchase #5 $5.86   6.729%   $6.2543   $6.25   $6.25   $6.30  
  Purchase #6 $101.51   9.605%   $111.26   $111.26   $111.25   $111.30  
  Purchase #7 $142.68   8.525%   $154.8435   $154.84   $154.85   $154.80  
  7 purchase totals = $651.16       $717.4827   $717.47   $717.50   $717.50