I run into this constantly on the job. Many people like the simplicity of a hard set policy; IE archive everything 30 days old or older. My argument to this is that quotas, while requiring a bit more planning and foresight, are eventually much more dynamic and easily managed in the long run.
My personal preference is to set the archive quota to use the warning limit. This way, it is seamless to the user as it prevents the notifications, and doesn't hit the send/receive limit either. Another pro to using the warning quota in Exchange is that because they don't see a warning, they don't misinterpret what is going on and try to archive it themselves. This is compounded if the ability to use PSTs has not been disabled via group policy. Hard set limits such as nothing in the last 7 days will help with potential user grief (especially if the slider on the quota limit tab is set toward size rather than date). Also setting the "Do not archive messages smaller than" setting is good to set so that it mitigates the frustration of having to pull down an archive that didn't really get any size savings anyways. Stubs tend to be around 2-3kb, so that part is a no brainer. I like setting it a bit higher due to the balance of overall size savings versus user acceptance. Again this is a culture based decision.
The quota limit tab is another place decisions come in to play. You'll want to set the percentage of the quota to begin archiving from to at least be less than what the person could receive in a day. That number also affects how far down to archive. The size versus message age debate is one of, surprise, culture. If your company receives a barrage of large images for viewing/editing, you'll obviously take different steps than if your business relies on email primarily as a quick messaging service.
Now to the dynamic part. The best part about this system is that you only need one policy. This means no messing with automation policies down the road, no messy clean ups, etc. Merely change the quota limit in Exchange for the mailstore or individual and viola. Powershell scripts in Exchange 2007 make this a very powerful solution!