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 $28.47   7.124%   $30.4982   $30.50   $30.50   $30.50  
  Purchase #2 $7.93   11.827%   $8.8679   $8.87   $8.85   $8.90  
  Purchase #3 $124.43   4.587%   $130.1376   $130.14   $130.15   $130.10  
  Purchase #4 $212.69   12.194%   $238.6254   $238.63   $238.65   $238.60  
  Purchase #5 $28.99   11.655%   $32.3688   $32.37   $32.35   $32.40  
  Purchase #6 $9.83   9.418%   $10.7558   $10.76   $10.75   $10.80  
  Purchase #7 $235.83   9.201%   $257.5287   $257.53   $257.55   $257.50  
  7 purchase totals = $648.17       $708.7824   $708.80   $708.80   $708.80