Some people will only trade $1's for $1 and $5's for $5 and so on.
This is because if one person is trading 2-3 $5 chips, in $1's, this could wipe out the other person's traders.

I personally will trade whatever works! If I want a $5 chip... I don't care if the person takes it in 10 x 50c chips (if I have them) if that makes the trade work for both of us.

Most people seem to prefer trading "common" chips straight across ($1's for $1's and $5's for $5's and so on)... or LE's for LE's... Hard Rocks for Hard Rocks...

Best thing is to ask them what guidelines they prefer.