Bed Raising?

Question:HI I have an old twin bed, with whells, and I have 4 bed rasiers, like these only ther black and round http://www.bedbathstore.com/woodbedriser...
What I was wondering is it safe to put these on a bed with wheels, they fit in the slot so I was just wondering is it safe.

I don't want to wake up on the floor one morning.

Answers:
I used these once and they are really unstable. I ended up getting rid of them for that exact same fear.