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 $69.76   13.811%   $79.3946   $79.39   $79.40   $79.40  
  Purchase #2 $45.44   13.196%   $51.4363   $51.44   $51.45   $51.40  
  Purchase #3 $143.89   9.861%   $158.079   $158.08   $158.10   $158.10  
  Purchase #4 $114.75   5.823%   $121.4319   $121.43   $121.45   $121.40  
  Purchase #5 $35.81   6.579%   $38.1659   $38.17   $38.15   $38.20  
  Purchase #6 $19.71   8.336%   $21.353   $21.35   $21.35   $21.40  
  Purchase #7 $18.53   3.131%   $19.1102   $19.11   $19.10   $19.10  
  7 purchase totals = $447.89       $488.9709   $488.97   $489.00   $489.00