Hey guys ... thanks for the replies. I've done more research on this, which I'll share here:
-- The computers, etc in the Tesla already draw somewhere in the range of 30-50W when the car is off. That's going to drain the 12v system (even before you add your 5W dashcam). What the Tesla does is the computers monitor the 12v system, and then if the voltage gets too low, a DC-DC converter charges the 12v battery (It's a 33AHr AGM lead-acid battery, similar but not exactly the same as an ICE 12v battery) from the main battery pack.
-- Adding a new device that is constantly draining power (even just 5W) means that the charging of the 12v battery will have to occur more frequently. That will decrease the expected life span (in time) for the battery.
-- There is no chance (unless your main battery is empty) that you'll "drain" the 12v battery. The computers won't allow it. So long as the 12v battery can take a charge, and you've got some juice in the main battery, you are okay.
-- Tesla does monitor the 12v line remotely as part of it's diagnostics, etc. Unlike the accessory power, there is a chance if you had a 12v battery die on you earlier than planned and under warranty, they could deny it as a warranty service item and you'd have to pay out of pocket. Just another thing to consider.
So in the end, like most things, it's a bit of a tradeoff. If you run off constant power, then you are providing additional battery load, and batteries are rated on their Charge Cycles in terms of expected life.
So, the difference is (between switched and always-on power) that the additional load on the 12v battery is either only while you are driving, or all the time.
In the past, I think the issue with the really, really short 12v battery lives (1 year or less) was HOW Tesla was charging the battery. From the data I've seen, it was charging the 12v battery 5 times a day and could hit it with up to almost 16v sometimes which is really terrible for a lead-acid battery.
I talked to my installer, who has done blackvue installs for Teslas and he recommended using the accessory (cig lighter) switched power. He said Tesla has recommended that to him, so that's what he recommends. However, I'm of the opinion that choice limits the dashcam's usefulness too much. I want it to monitor the car while it's parked and having it always on is better for enabling cloud data upload, etc.
Final thing is that the Tesla is already drawing a decent amount of power from the 12v battery all the time. You aren't doing anything that the Tesla isn't already doing by routing a dashcam to always-on power. It's just adding another item to the list.