Generally, I think everyone has been "guilty" of not trading up at one point or another. For the most part, I carry cheap little American flag pins as my swag, and as such, I generally leave and do not take.
When my boy is with me though I do have a reserve of toys to trade out for the various trinkets he might want to claim. I do not know if my trade is equal, or if I am trading up, but I try to get close. The guiding principal is that one should not take all the nifty articles others have placed and leave behind the Ubiquitous Ball of String in its place.