Yes it's good to have rules on how far one wants to go down the maintenance rabbithole. I usually set the limit at whether I'll have to buy an expensive tool just to do the service (e.g. one of those tools to clean the valves on these GDI engines) or whether the job requires removing more than N things (where N = 2 or so); if I have to disconnect a bunch of ancillary stuff just to replace a starter (or the back spark plugs in a lot of transversely-mounted V6 engines), then to the pro mechanic it goes!