I never read them, and they only serve to aggravate me. I know I have a bounty out there, and I want to let it run the full 7 days.
This is especially true on sites with a lot of bounties, such as MSO or SO, where much of the activity on bountied questions seems to happen in the last 24 hours.
I already feel spammed by my SE inbox sending me multiple notifications about my bounties expiring, and I get notified when I get new responses to the bounty and obviously didn't choose to award the bounty then.
I wouldn't mind an email after the bounty has expired and is in the grace period, as then it has served its purposes and should definitely be awarded, however receiving an unasked for email about a bounty I know exists and that I don't want to award yet is very irritating.
You guys are usually great about only sending out emails that I would actually be interested in reading, and being spammed with things like bounty notices expiring only dilutes that value.
So please, either make the "your bounty is ending soon" email an opt-in feature, or send it out after the bounty has run it's full course and is in its grace period.
In response to comments below, I'd actually like to suggest that you let users specify from a dropdown when they'd like to be reminded.
This could default to the current behavior (being notified 1 day before it expires), and users that care to change it (such as me!) could do so :)