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 $239.63   11.871%   $268.0765   $268.08   $268.10   $268.10  
  Purchase #2 $7.73   3.508%   $8.0012   $8.00   $8.00   $8.00  
  Purchase #3 $237.41   10.301%   $261.8656   $261.87   $261.85   $261.90  
  Purchase #4 $83.93   6.435%   $89.3309   $89.33   $89.35   $89.30  
  Purchase #5 $31.76   8.621%   $34.498   $34.50   $34.50   $34.50  
  Purchase #6 $170.57   3.363%   $176.3063   $176.31   $176.30   $176.30  
  Purchase #7 $220.30   13.239%   $249.4655   $249.47   $249.45   $249.50  
  7 purchase totals = $991.33       $1,087.5440   $1,087.56   $1,087.55   $1,087.60