I would consider a large goal of creep spread is to connect bases, either your bases or your base to your opponents base. So width of creep is less important than length.
Anyways creep from a tumor can only spread to normal terrain on the same height. So it doesn't spread to impassible/unmovable terrain, or high ground/low ground. Since creep can't spread to these area's, this means that if you build near them creep will spread more quickly. So if you build a creep tumor right next to a cliff the creep will fill out faster than if you built it on open ground. Since the tumor will see that it can't build on the illegal spaces and creep the next legal location.
This means that if you want to connect two bases as fast as possible you should build as close to cliffs as possible. Since the creep will build more forward instead of sideways.
The 2 creep's were started at same time, the one on the left is next to a wall and the one on the right is on open ground. Since the one on the wall has to spread less creep it spread faster.
This is a replay of me using this trick
Creep Spread Replay
Edit: 1 potential problem with this trick is that creep can spawn on unconnected high ground if there is creep there, which means that if you are hugging a cliff that starts with a ramp your creep will follow you all the way up the ramp and along the top of the cliff as you hug the wall. In order to prevent this you would have to build the first creep tumor away from the ramp then hug the wall afterwords to prevent creep from going to high ground and slowing creep spread.