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 $239.26   12.476%   $269.1101   $269.11   $269.10   $269.10  
  Purchase #2 $132.36   13.474%   $150.1942   $150.19   $150.20   $150.20  
  Purchase #3 $241.52   8.754%   $262.6627   $262.66   $262.65   $262.70  
  Purchase #4 $240.69   11.157%   $267.5438   $267.54   $267.55   $267.50  
  Purchase #5 $181.63   6.169%   $192.8348   $192.83   $192.85   $192.80  
  Purchase #6 $138.70   5.587%   $146.4492   $146.45   $146.45   $146.40  
  Purchase #7 $211.82   3.328%   $218.8694   $218.87   $218.85   $218.90  
  7 purchase totals = $1,385.98       $1,507.6642   $1,507.65   $1,507.65   $1,507.60