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 $6.52   3.242%   $6.7314   $6.73   $6.75   $6.70  
  Purchase #2 $78.75   11.746%   $88   $88.00   $88.00   $88.00  
  Purchase #3 $222.26   12.215%   $249.4091   $249.41   $249.40   $249.40  
  Purchase #4 $99.56   9.535%   $109.053   $109.05   $109.05   $109.10  
  Purchase #5 $41.78   13.151%   $47.2745   $47.27   $47.25   $47.30  
  Purchase #6 $108.48   8.256%   $117.4361   $117.44   $117.45   $117.40  
  Purchase #7 $20.44   9.298%   $22.3405   $22.34   $22.35   $22.30  
  7 purchase totals = $577.79       $640.2446   $640.24   $640.25   $640.20