I’ve concluded that a foolproof way is to use the host file to point the client’s domain to local host in my own machine. That way it doesn’t matter if the URLs are relative or not, though I always use relative paths in URLs.
At least when developing a CMS site you don’t have to search and replace the database for hard-coded URLs before uploading to the live server. (In my experience Wordpress and the like might save absolute URLs where you least expect it.)
If the host file option is impracticable I name the local host domain the same as the live but miss-spell the top level, e.g: .com to .cox to make the search and replace in the databases safe.
That’s my two cents in this matter. 