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 $20.26   11.702%   $22.6308   $22.63   $22.65   $22.60  
  Purchase #2 $117.86   8.186%   $127.508   $127.51   $127.50   $127.50  
  Purchase #3 $88.93   9.260%   $97.1649   $97.16   $97.15   $97.20  
  Purchase #4 $142.59   10.225%   $157.1698   $157.17   $157.15   $157.20  
  Purchase #5 $86.88   11.538%   $96.9042   $96.90   $96.90   $96.90  
  Purchase #6 $85.77   13.397%   $97.2606   $97.26   $97.25   $97.30  
  Purchase #7 $39.67   11.511%   $44.2364   $44.24   $44.25   $44.20  
  7 purchase totals = $581.96       $642.8747   $642.87   $642.85   $642.90