Hi Kervin,
From my experience, when I got my new mattress, we slept on it which was placed on the floor. When my bedroom set came in later, we placed the mattress on the bed frame but the bed frame was not pushed against the wall..leaving a little gap there.
An Chuang date is usually chosen about 1-3 days before AD as we are not allowed to touch or sleep on the bed after that..For Good Luck and Blessing, I guess. In fact, to be on the safe side, we locked the room. On An Chuang day itself, my parents in law pushed the bedframe against the wall, changed the bedsheet and place those items that we bought. Can't remember exactly what they are, but I think it consists of red dates, longan, ruler, oranges, coins and more....
It is really depends.. Everyone has their own idea. We are just following the tradition in a more flexible way. What I can suggest is...you may still sleep on the mattress but instead of placing them on the platform bed, you may need to place it at some other room on the floor. On An Chuang day, new bedsheet is a must. If you are not sure, you can just go to shops @ Chinatown and ask for their advice.
Hope this helps