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 $228.66   13.569%   $259.6869   $259.69   $259.70   $259.70  
  Purchase #2 $13.45   13.272%   $15.2351   $15.24   $15.25   $15.20  
  Purchase #3 $50.75   6.590%   $54.0944   $54.09   $54.10   $54.10  
  Purchase #4 $188.26   5.515%   $198.6425   $198.64   $198.65   $198.60  
  Purchase #5 $83.30   12.568%   $93.7691   $93.77   $93.75   $93.80  
  Purchase #6 $72.63   13.795%   $82.6493   $82.65   $82.65   $82.60  
  Purchase #7 $172.48   13.301%   $195.4216   $195.42   $195.40   $195.40  
  7 purchase totals = $809.53       $899.4989   $899.50   $899.50   $899.40