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 $225.36   13.723%   $256.2862   $256.29   $256.30   $256.30  
  Purchase #2 $240.55   8.488%   $260.9679   $260.97   $260.95   $261.00  
  Purchase #3 $166.85   8.762%   $181.4694   $181.47   $181.45   $181.50  
  Purchase #4 $126.26   7.713%   $135.9984   $136.00   $136.00   $136.00  
  Purchase #5 $248.96   9.506%   $272.6261   $272.63   $272.65   $272.60  
  Purchase #6 $239.46   12.549%   $269.5098   $269.51   $269.50   $269.50  
  Purchase #7 $130.49   8.506%   $141.5895   $141.59   $141.60   $141.60  
  7 purchase totals = $1,377.93       $1,518.4473   $1,518.46   $1,518.45   $1,518.50