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 $229.27   8.379%   $248.4805   $248.48   $248.50   $248.50  
  Purchase #2 $232.46   6.619%   $247.8465   $247.85   $247.85   $247.80  
  Purchase #3 $95.26   3.187%   $98.2959   $98.30   $98.30   $98.30  
  Purchase #4 $183.56   6.650%   $195.7667   $195.77   $195.75   $195.80  
  Purchase #5 $150.60   3.695%   $156.1647   $156.16   $156.15   $156.20  
  Purchase #6 $244.77   6.412%   $260.4647   $260.46   $260.45   $260.50  
  Purchase #7 $30.41   5.149%   $31.9758   $31.98   $32.00   $32.00  
  7 purchase totals = $1,166.33       $1,238.9948   $1,239.00   $1,239.00   $1,239.10