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 $140.62   12.242%   $157.8347   $157.83   $157.85   $157.80  
  Purchase #2 $57.83   8.218%   $62.5825   $62.58   $62.60   $62.60  
  Purchase #3 $115.57   3.210%   $119.2798   $119.28   $119.30   $119.30  
  Purchase #4 $54.38   12.653%   $61.2607   $61.26   $61.25   $61.30  
  Purchase #5 $217.61   13.621%   $247.2507   $247.25   $247.25   $247.30  
  Purchase #6 $159.41   5.316%   $167.8842   $167.88   $167.90   $167.90  
  Purchase #7 $247.72   9.594%   $271.4863   $271.49   $271.50   $271.50  
  7 purchase totals = $993.14       $1,087.5789   $1,087.57   $1,087.65   $1,087.70