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 $54.41   10.831%   $60.3031   $60.30   $60.30   $60.30  
  Purchase #2 $168.75   8.482%   $183.0634   $183.06   $183.05   $183.10  
  Purchase #3 $213.59   10.856%   $236.7773   $236.78   $236.80   $236.80  
  Purchase #4 $237.43   10.722%   $262.8872   $262.89   $262.90   $262.90  
  Purchase #5 $47.32   3.448%   $48.9516   $48.95   $48.95   $49.00  
  Purchase #6 $33.79   5.498%   $35.6478   $35.65   $35.65   $35.60  
  Purchase #7 $11.35   12.358%   $12.7526   $12.75   $12.75   $12.80  
  7 purchase totals = $766.64       $840.3830   $840.38   $840.40   $840.50