How old are they? Not all chickens start roosting at the same age, but some keep sleeping on the floor until they are a lot older than you’d think, especially if you have mature chickens in the coop and on the roosts.
Perching is when they get up on something during the day to look around and survey their domain. Lots of chicks do that before they start to roost. Roosting is when they spend the night on the roosts. I’m pretty sure roosting is what you are talking about, not just perching, but I like to make sure.
Silkies cannot fly which can make them getting up or down a bit of a problem. Getting down can be a bigger problem than getting up. They need something pretty low to jump onto and off of or a ramp or intermediate steps of some sorts. But they are individuals. Some are content to sleep on the floor their entire lives. Others like to roost. It does not hurt them either way.
I don’t know what your physical set-up is but I’d suggest you give them an easy way up and down then let them decide. They’ll be OK either way.