I would think good sense judgement wins it out here.
If the question is, "will doing my own service intervals void my warranty", the answer is most likely "no". This assumes the work was done correctly. Best to keep receipts and good records, and allow the dealer to perform any technical inspections, if there are any (my BMW needed value inspections/adjustments, not something I would normally perform).
As for aftermarket modifications, the best course of reason is that if a modification is made that can directly result in a failure, it will most likely come under heavy scrutiny. This means, if you jack with something and it fails, be prepared. A warranty cannot be voided in total for a isolated modification. So if you make aftermarket modifications to your suspension, and you have suspension issues down the road, you're liable to be rejected. However, if you have a transmission issue, they cannot reject it because you made a suspension modification - unless the two can be linked.
All said, dealers and guarantors run their businesses as such, and you will find all sorts of types out there - good and bad. My advice is to find a dealer you trust, build a good relationship with them by visiting them, purchase all of your service items from them so they have the records, keep very good records of everything you do, and if you have issues, they will be more likely to stand behind you.