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 $61.62   7.337%   $66.1411   $66.14   $66.15   $66.10  
  Purchase #2 $129.78   13.682%   $147.5365   $147.54   $147.55   $147.50  
  Purchase #3 $132.26   10.560%   $146.2267   $146.23   $146.25   $146.20  
  Purchase #4 $167.91   3.285%   $173.4258   $173.43   $173.45   $173.40  
  Purchase #5 $161.80   7.597%   $174.0919   $174.09   $174.10   $174.10  
  Purchase #6 $142.38   8.792%   $154.898   $154.90   $154.90   $154.90  
  Purchase #7 $105.33   11.191%   $117.1175   $117.12   $117.10   $117.10  
  7 purchase totals = $901.08       $979.4375   $979.45   $979.50   $979.30