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 $20.75   4.425%   $21.6682   $21.67   $21.65   $21.70  
  Purchase #2 $32.71   9.384%   $35.7795   $35.78   $35.80   $35.80  
  Purchase #3 $161.47   8.686%   $175.4953   $175.50   $175.50   $175.50  
  Purchase #4 $105.62   8.485%   $114.5819   $114.58   $114.60   $114.60  
  Purchase #5 $40.51   8.601%   $43.9943   $43.99   $44.00   $44.00  
  Purchase #6 $137.80   11.659%   $153.8661   $153.87   $153.85   $153.90  
  Purchase #7 $85.37   11.724%   $95.3788   $95.38   $95.40   $95.40  
  7 purchase totals = $584.23       $640.7641   $640.77   $640.80   $640.90