If you hang around here much, I'm sure you know I can't pass up a Big Bend thread.
March is probably the busiest time of the year due to the college kids on Spring Break so I'd go with the reservations. Having said that, after arriving, we've frequently found a better site than the one we'd reserved and have always been allowed to switch without any hassle. You may want to take your chances but for us, after a 2K mile drive, we want to know we've got a spot.
If you can't reserve a site in the Basin for your entire stay, don't hesitate to lock in a night or two at RGV. Its a neat campground and there's a lot to do at that end of the Park - Boquillas Canyon, Hot Springs, Old Ore Road (if you have 4WD). We also like Cottonwood on the west side - Its more primitive (no generators and pit toilets only) but usually less crowded with more wildlife. Its also more convenient for visits to Terlingua, Study Butte and if you have an extra day, Big Bend Ranch State Park (beautiful drive along the river).
The drive from the main road into the Basin is about four miles with three or four hairpin curves over the last section. Its not too difficult for smaller trailers and poses no problem at all for a single vehicle.
We like the Basin because of its proximity to three of the signature trails in the Park - The Window, Lost Mine and South Rim (The last is better done as an overnighter for anyone who spends as much time behind a desk as I do). It also has a decent restaurant if you want a break from cooking.
If you have time and can work it out, spending time at each campground is a great way to experience Big Bend. That's what we'll be doing over Easter. If anyone else has plans to be there around then, please drop by and say 'hi'.
Al & Cindy