I went to the Dr's. last month, 3 months before we wanted to TTC.
They checked my bloods for general health and also for immunity against MMR etc. Like the PP, I'm not immune to Rubella despite having all my childhood immunisations.
As I'm also at Uni, my risks for rubella are slightly increased so I was advised to have the MMR jab again. This means we now can't TTC for a total of 4 months (this was my Dr's. advice - there is conflicting advice out there to research).
For me, the visit was worth it for that alone, although plenty of people have happy, healthy pregnancies and babies without a GP visit beforehand.
The Dr. also checked I was taking pre-pregnancy vitamins with folic acid, and gave general advice re. healthy eating, exercise and lifestyle.
Even if nothing else, it gave me an opportunity to ask a few things and if there had been anything else it would have come up.
I'm going back in a few weeks to see if the immunity for Rubella has taken this time. Have a couple of extra questions like certain over the counter meds/face creams etc, so It's really just an opportunity to raise any queries.
I'd also read (probably on MN) about having a dentist checkup as some procedures may be off limits during pregnancy, and because pregnancy can apparently wreck your gums.
Luckily all was well, so I just had a regular exam plus scale and polish. I think there's some research that suggests there may be some nasties in tooth plaque etc that can have some impact down the line - either way, it's nice to have them properly cleaned etc.
Having said all that, there are many, many women who have perfectly happy and healthy pregnancies without any of these pre-conception check ups so if it's not your style then I wouldn't worry about it!