‘Long Contractor Level’
A long level will ensure that your rails are level to maintain uniformity on the fence line. Absolute accuracy is not a must, but the better job you do checking, the better the fence’s appearance will be when complete.
At least a 36 inch long level is best, but the longer you use, the more accuracy you will have and the easier the job will be. The level of quality is more of a personal budget preference for this job.