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 $250.58   6.154%   $266.0007   $266.00   $266.00   $266.00  
  Purchase #2 $248.51   4.701%   $260.1925   $260.19   $260.20   $260.20  
  Purchase #3 $126.63   7.376%   $135.9702   $135.97   $135.95   $136.00  
  Purchase #4 $55.88   4.656%   $58.4818   $58.48   $58.50   $58.50  
  Purchase #5 $35.99   3.235%   $37.1543   $37.15   $37.15   $37.20  
  Purchase #6 $7.38   4.637%   $7.7222   $7.72   $7.70   $7.70  
  Purchase #7 $111.40   4.364%   $116.2615   $116.26   $116.25   $116.30  
  7 purchase totals = $836.37       $881.7832   $881.77   $881.75   $881.90