Hummmmmm... I've done it both ways. I like to see firsthand the courses I've heard were difficult, so I know when "the worst is over". I toured the Baltimore Marathon course the day before and absolutely freaked out -- not because of the hills (which are plentiful) but because I kept imagining myself alone (I'm a back-of-the-pack walker) in some iffy-looking neighborhoods. On race day, everything was perfect -- loads of police and course support and some really wonderful scenery. I had nothing to worry about.
The flip side of the Baltimore Marathon tour was that the race director who led the tour was also accompanied by a professional tour group leader, so he told us all about the history behind some of the areas we were running through. It provided an entirely new aspect to the course. Also, I tend to think even if you see the course beforehand in a car or bus, it's not like what you notice as you run/walk through, so I still had plenty to entertain me.
I suppose my bottom line is I can't give you a simple yes/no answer. I think, like most things in life, some good and bad comes out of touring the course beforehand. For my part, I'd say the good outweighs the bad, but your experience may prove different. I've certainly done many more marathons where I had no time to tour the course before the race, and I enjoyed them as well.
I'd encourage you to go with your gut feeling. If you like planning ahead, go. If you like surprises, don't go.
I am sure your first marathon experience will be memorable in any case.