I would say it depends on the time of year you are going. We drive up & down I95 & hardly ever make reservations ahead of time. Be sure to stop at the state welcome centers & pick up the hotel coupon books. We have frequently found good values reading through them.
Two years ago we went at the end of April. The Saturday we left turned out to be the Saturday after Easter - very few hotel rooms to be had by the time we wanted to stop. We ended up at a Best Inn, near the south part of GA, which we renamed the WORST Inn - no hot water, no a-c, no heat. We did end up getting our money back the next mroning, but never again!
We have had good luck at the Hampton Inns in Florence, SC & Richmond Hills, GA & Clark's Inn in Santee, SC. We used to stay a lot in Hardeeville, SC, but things seemed to have changed in the area - just a gut feeling, not something I can put my finger on. SO, if you are going during a hoiday period you might want to make a reservation or perhaps stop a bit earlier to find a decent room.
Oh, the one time we did make a reservation ahead of time was Thanksgiving Saturday a couple of years ago - it rained so hard for so long that we never made it that Hampton Inn. We called & changed to another Hampton Inn.