My solution is an incredibly simple paper-based filing system inspired by David Allen's Getting Things Done, and it only relies on a couple of really simple heuristics:
- Files are stored alphabetically. This is straight from GTD is seems duh obvious, but my previous system grouped warranties in one drawer, insurance in another, etc. In practice it took much longer to find things than it does now, but now there's no hierarchy: everything is at the same level.
- Name folders the first thing that comes to mind, because that's the same thing you're going to think of when you need the info again. So if you think "phone" instead of "iPhone" or "stove" instead of "range" that's obviously what you should file stuff under.
- One thing per folder. This gives you more freedom in naming (see above) and means you have to wade through less content to find that one thing.
This isn't as sexy as scanning, but it's a lot quicker, which means you'll actually do it.
There's a sneaky second part to your question, which has to do about separating reference materials from actions. Receipts, warranties, and manuals are references. You don't plan on needing them, but want them around in case something breaks or whatever. Those get filed.
The service schedule and special offers are something else. If the service schedule has regular maintenance that I need to proactively schedule, then I'm going to put it into my action tracking system and tag it to automatically enter my context lists when it's time to make the appointment (the electronic equivalent of a tickler file). If it's a coupon I probably have several tasks: deciding if I want to buy what's on special, researching prices, etc. So perhaps I file the coupon for reference, but not without capturing the actions that will result in me using it or deciding to throw it away.
Lastly, you ask about maintaining your files to get rid of the junk. Again, less is more. I don't recommend a regular task to clean out files. First, that's boring work, so you're likely to put it off, even if it's in your actions list. Second, it's needless busy work; who cares if that paper is sitting there 99.9% of the time?
I defer purging files until I need to do so. For example, you get a new microwave and come across the old manual when filing the new one. That's the time to throw it out. In other words, get rid of old files as you come across them; this way you're effectively triaging your purge time to toss the things that actually get in your way. (The only time I've done a global purge when was my files got too big for my cabinet. I purged until I had the room I needed and then stopped.)
If you wanted to get real ninja, you could use that tickler file to remind yourself to throw away warranties when they expired, but honestly, that seems like overkill.