It depends on the situation. If I'm working with a brand new person, I'll take falls so they can see how something is supposed to work. If I'm working with someone who is more advanced, I only fall when they "make me" by their technique.
That having been said, that doesn't mean that you fight and claw with each technique so as not to have to fall. If the nage takes your balance and there is legitimate fall, you should fall. Don't give anything but keep in mind that your body will learn how not to fall so it takes some discernment on your part and lots of sensitivity.
BTW, at level are you corrently working?