When deploying a SharePoint 2010 farm, what is the recommended distribution of services on farm servers? For example, given a small farm having 2 WFE servers and 1 App server (and one SQL server): should all services be setup on the app server, or can some of them also be setup on the WFEs? Are there some that should only be run on an APP server and others that should only be run on a WFE server? I've searched through the literature, and though I find a large amount of Microsoft documentation with respect to services running in a farm, there is next to no documentation that discusses what services to setup and configure on which server.
Take for example BCS, Access, Excel and word automation: does it matter whether these services are setup on a WFE as opposed to an APP server?
From my read of the Microsoft documentation, such as the SharePoint 2010 Planning Guide, all services should be setup on the App server and the WFE server(s) should only run th web service, load balancing and query services. However, in practice, I am finding customers who have setup services on both App and WFE servers. I would like to know if there is documentation discussing recommended service distribution among the servers on a farm and also know how other admins have approached this topic.