We probably shouldn't allow negative padding.
Yes you should.
It is sometimes very useful. For example, it is the only (well, easiest) way to delay the start of a subscription recording if there has been a last minute delay of a program start, where the EPG isn't going to be updated. If say a reality or sports show runs late, as they tend to do, before a program scheduled to be recorded, a user could add a negative Pre-Padding to delay the recording start time, and more Post-Padding to capture the end of the program.
Of course a user can now edit the start time for a program in the EPG/Guide data, but they can't edit the program Duration to catch an overrun, so Post-Padding is still required. Changing the start time requires setting up a view in Standard View that shows the EPG data, and then editing the data before the program start time. That may have consequences in the Guide display and maybe otherwise, since it will create program overlaps.
Negative padding, particularly Pre-padding, is a useful feature. Users just need to understand how padding works, in that a positive Pre-Padding means start the recording early. All PVRs I have used, hardware and software, use that convention. i.e. Positive Pre-Padding means start early, and positive Post-Padding means continue recording for extra time after the program completion time.
Of course there are programmatic ways to get around the above issue, but I suspect that a "Delay recording start time by X minutes" function would still best be implemented using Pre and Post Padding, rather than changing EPG data, since that data would be overridden in the next EPG update. A nice feature would be to collect and use the "Now & Next" data broadcast with programs to adjust recording start times "on the fly" based on minor scheduling changes. That is a big job though, and probably region specific, like EPG data.
There was a post regarding this very recently, here:
http://yabb.jriver.com/interact/index.php?topic=103863.msg721832#msg721832... I see both Glynor and Yaobing beat me to it. I should write less.