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 $243.91   6.488%   $259.7349   $259.73   $259.75   $259.70  
  Purchase #2 $183.96   3.789%   $190.9302   $190.93   $190.95   $190.90  
  Purchase #3 $54.42   8.155%   $58.858   $58.86   $58.85   $58.90  
  Purchase #4 $127.41   6.174%   $135.2763   $135.28   $135.30   $135.30  
  Purchase #5 $5.88   6.872%   $6.2841   $6.28   $6.30   $6.30  
  Purchase #6 $65.74   11.378%   $73.2199   $73.22   $73.20   $73.20  
  Purchase #7 $13.41   12.408%   $15.0739   $15.07   $15.05   $15.10  
  7 purchase totals = $694.73       $739.3773   $739.37   $739.40   $739.40