Sounds like folks have covered the bases. However, you don't have to use a res and can use a T-line also, in which case you still have to make sure the pump has a ready supply of water when filling up. This is why people that use t-lines usually install them right at the pump inlet.
You can put a res other places besides at the pump inlet, but you have to know what you are doing to make it work, so it is the norm to make things work and make them easy to keep the res at the pump inlet. In some builds it can become very difficult to put the res at the pump inlet.