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 $38.72   10.744%   $42.8801   $42.88   $42.90   $42.90  
  Purchase #2 $40.42   3.474%   $41.8242   $41.82   $41.80   $41.80  
  Purchase #3 $114.27   8.722%   $124.2366   $124.24   $124.25   $124.20  
  Purchase #4 $12.99   8.330%   $14.0721   $14.07   $14.05   $14.10  
  Purchase #5 $16.76   11.698%   $18.7206   $18.72   $18.70   $18.70  
  Purchase #6 $173.63   10.807%   $192.3942   $192.39   $192.40   $192.40  
  Purchase #7 $90.81   7.326%   $97.4627   $97.46   $97.45   $97.50  
  7 purchase totals = $487.60       $531.5905   $531.58   $531.55   $531.60