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 $242.41   12.622%   $273.007   $273.01   $273.00   $273.00  
  Purchase #2 $182.85   8.813%   $198.9646   $198.96   $198.95   $199.00  
  Purchase #3 $144.78   5.186%   $152.2883   $152.29   $152.30   $152.30  
  Purchase #4 $168.45   7.656%   $181.3465   $181.35   $181.35   $181.30  
  Purchase #5 $95.25   3.243%   $98.339   $98.34   $98.35   $98.30  
  Purchase #6 $104.90   13.867%   $119.4465   $119.45   $119.45   $119.40  
  Purchase #7 $217.83   7.367%   $233.8775   $233.88   $233.90   $233.90  
  7 purchase totals = $1,156.47       $1,257.2694   $1,257.28   $1,257.30   $1,257.20