I noticed that still several customers use marketing attributes to store opt-in / opt-out information they receive from their customers.
Consider to use marketing permissions instead.
This functionality is available in CRM 2007, and higher.
The advantage is that segmentation and campaign execution consider marketing permissions in standard.
Besides you can define opt-in / opt-in regulations per country, region, and even per communication channel, for example e-mail. This is done in Customizing.
Check out SAP Note 1574230 for details.