If the subdomains are added to WMT google will know about it. if you are designing sites for clients and putting them on your site as subdomains it behooves you to make sure 100% that their dev sites are not being seen by Google. It's duplicate content and your subdomain is the original source of this content. Looks unprofessional too 
a) verify any subdomain you are creating for a client in WMT
b) block it in robots.txt and noindex nofollow all pages globally
c) for the ones that are already indexed, go into google WMT and go into that subdomain account and request removal of the site in Googles index. This will remove the indexing for that subdomain only don't worry it won't remove your main site from the index.