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 $21.80   3.424%   $22.5464   $22.55   $22.55   $22.50  
  Purchase #2 $46.46   12.123%   $52.0923   $52.09   $52.10   $52.10  
  Purchase #3 $6.35   8.365%   $6.8812   $6.88   $6.90   $6.90  
  Purchase #4 $8.41   7.562%   $9.046   $9.05   $9.05   $9.00  
  Purchase #5 $227.29   6.515%   $242.0979   $242.10   $242.10   $242.10  
  Purchase #6 $47.54   7.752%   $51.2253   $51.23   $51.25   $51.20  
  Purchase #7 $157.50   5.812%   $166.6539   $166.65   $166.65   $166.70  
  7 purchase totals = $515.35       $550.5430   $550.55   $550.60   $550.50