msycamore said:

I would not call my subs safe from TV-overscan.

For commercial releases, the "Action Safe" zone is 10% inside the video border, and the "Title Safe" zone is 20% inside the video border. Since flatscreens have less overscan than crt's, HD productions can use a smaller margin of error, say 5% for Action Safe and 10% for Title Safe.

Since there is a possibility a customer will play an anamorphic dvd on a 4:3 tv and choose to crop, the "Title Safe" zone would be, to ensure proper display in this worst case scenario, 10% inside the 4:3 portion of the video.

All of this is to guarantee, for commercial releases, that any words displayed in the video will be readable for all customers.

Since this is not a commercial release, feel free to do whatever you think reasonable. I'd say you can assume the viewer is using a 16:9 screen (ergo, allow your subtitles to fail for the 4:3 crop scenario). But I think it would still be good to observe the 10% parameter for Title Safe area inside the 16:9 frame, since this is kind of a standard, and a reasonable one at that.

The fact is that overscan is different on each device.

For your enjoyment, here is a graphic with the more conservative 4:3 safe area inside a 16:9 field displayed:
http://i206.photobucket.com/albums/bb41/joemorelli99/1080_sqpix_4x3_ttlsf.jpg

Here's a graphic with the 16:9 safe areas for 16:9 (would fail for a 4:3 scenario)
http://www.dvinfo.net/forum/final-cut-suite/144686-4-3-title-safe-16-9-comp.html

Here's a little discussion of the issues and a graphic in AfterEffects
http://toolfarm.com/reviews/aecs4.html

Here is more than anyone should ever know about safe zones:
http://provideocoalition.com/index.php/cmg_blogs/story/unsafe_areas/P1/