The built in speakers will be fine to start with. However, a true center channel speaker designed for the job will always sound much better, as it is "voiced" to reproduce the dialogue with much better definition and clarity by most good Loudspeaker Companies.
I have a Hitachi 65G500 65" projection TV. I was thinking about adding some components to start out a simple Dolby 5.1 system. Is there any problem using the two built in speakers in my TV cabinet as the center channel? Then I can add two front, to side and a subwoofer and have a component system to start out with.