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 $246.79   9.688%   $270.699   $270.70   $270.70   $270.70  
  Purchase #2 $225.58   11.242%   $250.9397   $250.94   $250.95   $250.90  
  Purchase #3 $159.26   6.157%   $169.0656   $169.07   $169.05   $169.10  
  Purchase #4 $169.87   4.555%   $177.6076   $177.61   $177.60   $177.60  
  Purchase #5 $122.29   12.597%   $137.6949   $137.69   $137.70   $137.70  
  Purchase #6 $167.43   4.771%   $175.4181   $175.42   $175.40   $175.40  
  Purchase #7 $158.34   11.428%   $176.4351   $176.44   $176.45   $176.40  
  7 purchase totals = $1,249.56       $1,357.8600   $1,357.87   $1,357.85   $1,357.80