I allow some, but if they start fighting over them or are getting hurt, I will put them up. I do have to say though, my kids don't really fight or hurt each other over them. There was a thread (I think on Friday), but I can't remember what it was about, that gave a link to a video on Youtube by Nannyde on how to have children play with push toys. It sounded like a good idea.
Oh here's the link:
http://www.youtube.com/watch?v=y1LyRILn4gQ