Not a bad idea, especially for new members who are not familiar with the 180 rule.
If they read the Terms of Service when signing up like they are supposed to, they would know about the 180 day rule before even diving in.
The concept of some type of notice about expiring offers isn't bad, but I agree e-mails wouldn't be the way to go. If anything, maybe something could be implemented on the section for 'Offers credited to your account'. Maybe add a list to the right of the 'Status' column called 'Days to Expiration' and have a countdown from 180 next to each offer. Once the countdown reaches 30 or 14 or whatever amount of days on the offer, the text could appear in red or something eye-catching, highlighting the warning of it soon expiring.
Or, maybe just some box in the account page named something like "Soon to Expire Offers", where offers within a designated amount of days of expiring show up in it with how many days until they expire listed next to each one.
I've never had a problem with this issue. However, for those who have, I could see this being at least a bit useful. If you complete an offer today, March 13th, it doesn't expire September 13th. At first thought it would. But, 180 days later would be September 9th. I can see how that could catch some off guard.