One could take the approach that even Helper Things have services that are Template based.
Templates represent either an asset, a helper, a utility, an entity type. So in accordance with that you would group your services per template which then spin into Things.
Probably one of the nice parts to have Services on a template is to be able to utilize the 'me' context vs. having to retrieve or pass that information.
However from resource usage perspective, the whole model is in memory so it is not more costly to read from other entities vs. within the context of the entity itself.
I believe ThingWorx fundamentals does cover some of the Best Practice approaches to this.