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 $246.48   13.331%   $279.3382   $279.34   $279.35   $279.30  
  Purchase #2 $5.99   10.658%   $6.6284   $6.63   $6.65   $6.60  
  Purchase #3 $85.36   8.555%   $92.6625   $92.66   $92.65   $92.70  
  Purchase #4 $13.77   4.761%   $14.4256   $14.43   $14.45   $14.40  
  Purchase #5 $85.72   12.807%   $96.6982   $96.70   $96.70   $96.70  
  Purchase #6 $213.91   11.186%   $237.838   $237.84   $237.85   $237.80  
  Purchase #7 $33.68   6.785%   $35.9652   $35.97   $35.95   $36.00  
  7 purchase totals = $684.91       $763.5561   $763.57   $763.60   $763.50