Presumably you are referring to power line converters?
Assuming that you are and that your summerhouse has it's own consumer unit, then the two ends of the link are not connected to the same ring main, this will not prevent them from working but it will make them more likely to fail every so often.
So, to answer your question; Yes, it's probably best to switch them off when they are not in use as it will act like a restart every time you power them back up and hence most probably reduce the likelihood of the link failing whilst it is in use.
