I've tried to solve this query several times before, but never really got anywhere. I'm in the process of swapping over to a Win 2012 IIS 8 server & the question has popped up again & I'd love to resolve it, so here goes:
Unless I give IUSR write permission to the content & includes folders, then I cannot upload stuff or install plugins, ditto the entire site directory in wwwroot when I come to upgrading the site.
I usually end up just giving iusr write access to the entire site as it saves any problems for customers who are trying to administer their sites.
I run regular backups (once per day) so have some disaster recovery in place, but I've never worked out whether or not this is 100% secure. I don't want a client to have to ring up each time they want to upgrade their site, but similarly I wanted to check that this is the correct way to go about doing things?