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 $20.86   7.847%   $22.4969   $22.50   $22.50   $22.50  
  Purchase #2 $40.57   13.720%   $46.1362   $46.14   $46.15   $46.10  
  Purchase #3 $168.46   8.873%   $183.4075   $183.41   $183.40   $183.40  
  Purchase #4 $111.32   11.110%   $123.6877   $123.69   $123.70   $123.70  
  Purchase #5 $11.49   10.110%   $12.6516   $12.65   $12.65   $12.70  
  Purchase #6 $81.30   8.649%   $88.3316   $88.33   $88.35   $88.30  
  Purchase #7 $146.69   8.558%   $159.2437   $159.24   $159.25   $159.20  
  7 purchase totals = $580.69       $635.9552   $635.96   $636.00   $635.90