Akinswi,
Do you mean you are removing the expander with the pin? They are a unit in the Lee FL dies, but the pin can be removed separately in most other brands. If so, this does leave the neck narrower than the collet die made it and that can make a difference in bullet pull. But if you are already using the collet die, you are already making a very good low-runout neck resizing job on the case and running it into the FL die is just overworking the neck unnecessarily.
Here is what I have done, and both approaches work very well, but each calls for another (but different) die:
Use the Lee Collet die for the neck and resize the case body (set the shoulder back) using a
Redding Body Die, which is like an FL resizing die except it is just a few thousandths wider than the case neck, and so resizes the rest of the case (the body) but does not touch the neck.
Use your FL sizing die without an expander (you can decap on the Collet Die without closing it hard or you can get a
Lee Universal Decapper) and then expand using a
Lyman M-die. You set the M-die up so it just opens the last little bit of the case mouth with its step profile just before the flare (you don't need to set it all the way to the flare for jacketed bullets; the flare is just for cast bullets). That little step lets you set the bullet upright in the case for seating and keeps it there while you run the press ram up so bullets don't enter the seating die at an angle. This causes them to pick up much less tip runout during seating, and this improves accuracy measurably.
If you own a Redding Competition Seater Die or a Forster Benchrest Seating Die, you can use the first method to very good effect, as these dies do a great job of straightening bullets on the fly. If you have only a standard seater or the Lee Dead Length Seating Die, using the second method is the best way to get very concentric ammunition.