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 $248.49   11.476%   $277.0067   $277.01   $277.00   $277.00  
  Purchase #2 $109.94   10.611%   $121.6057   $121.61   $121.60   $121.60  
  Purchase #3 $136.63   7.570%   $146.9729   $146.97   $146.95   $147.00  
  Purchase #4 $66.73   8.473%   $72.384   $72.38   $72.40   $72.40  
  Purchase #5 $163.90   5.732%   $173.2947   $173.29   $173.30   $173.30  
  Purchase #6 $248.77   8.748%   $270.5324   $270.53   $270.55   $270.50  
  Purchase #7 $199.68   13.529%   $226.6947   $226.69   $226.70   $226.70  
  7 purchase totals = $1,174.14       $1,288.4911   $1,288.48   $1,288.50   $1,288.50