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 $212.80   12.598%   $239.6085   $239.61   $239.60   $239.60  
  Purchase #2 $177.50   12.639%   $199.9342   $199.93   $199.95   $199.90  
  Purchase #3 $70.90   5.437%   $74.7548   $74.75   $74.75   $74.80  
  Purchase #4 $64.41   13.773%   $73.2812   $73.28   $73.30   $73.30  
  Purchase #5 $188.57   11.694%   $210.6214   $210.62   $210.60   $210.60  
  Purchase #6 $196.25   3.793%   $203.6938   $203.69   $203.70   $203.70  
  Purchase #7 $78.54   11.450%   $87.5328   $87.53   $87.55   $87.50  
  7 purchase totals = $988.97       $1,089.4267   $1,089.41   $1,089.45   $1,089.40