What breeds are you talking about? Some chickens can't fly, like Silkies or Frizzles. I don't know about Jersey Giants, how wekk they fly. I've never owned any of those.
What does your coop look like? Do they have room to spread their wings and fly?
How old are they? I've seen two week old chicks perform some amazing flying tricks, but maybe real, real young ones could use a ramp. They just won't need it for long.
There are exceptions but about any chicken can easily get up on a 4 foot high roost if they have enough room to work with. I've seen my full sized fowl fully grown hens clear a 4' high fence starting from the ground and have a foot to spare.
There is a whole lot about ramps on this forum. There are a few instances where the are needed, but usually they really are not needed.