I'm about level 40 with my character, and still in Act 2. I'm wondering if the storyline (Act 3 and Epilogue) finish pretty much exactly at the time you get the 50th level, or does the storyline continues for a couple of hours of gameplay after you reached the level cap?
I've dinged 50 during the final class storyline mission on both of my level 50's, but based on conversations with others, that seems to be a lucky coincidence.
The class storyline continues through Corellia, the final planet, which is tuned for levels 48-50, and generally ends with a series of visits to class specific locales (think space stations, ships, obscure planet only for your class, that sort of thing), tuned for a max level or near max level character.
That said, you could easily hit 50 well before doing it if you have a lot of 'extracurricular (PvP, FP's, bonus series, space missions, etc.)' XP coming in, or get there much sooner with a high level buddy tagging along to kill things for you, so YMMV.
I have done both, hit 50 before finishing the class quest and finished the class quest before 50.