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 $212.62   12.847%   $239.9353   $239.94   $239.95   $239.90  
  Purchase #2 $98.34   13.623%   $111.7369   $111.74   $111.75   $111.70  
  Purchase #3 $154.26   13.611%   $175.2563   $175.26   $175.25   $175.30  
  Purchase #4 $116.49   13.303%   $131.9867   $131.99   $132.00   $132.00  
  Purchase #5 $117.64   11.457%   $131.118   $131.12   $131.10   $131.10  
  Purchase #6 $190.88   7.793%   $205.7553   $205.76   $205.75   $205.80  
  Purchase #7 $160.36   12.621%   $180.599   $180.60   $180.60   $180.60  
  7 purchase totals = $1,050.59       $1,176.3875   $1,176.41   $1,176.40   $1,176.40