Basically.. Follow the manufactures instructions. They will specify wether or not you can cut to any length or specific increments. If it says to cut only on the line I would probably only cut it on the line.

When I build my rack I ordered tape from beanfarm also. But I chose to "snake" my tape throughout the rack rather than cut it at all. A 4 shelf rack that hold either 8x 15qt sterilite tubs or 4x 32 quarts took 10' of flexwatt and only one connection.

BUT I believe... ALL the flexwatt style heat tape MUST HAVE A THERMASTAT. Otherwise they will melt.