You are correct, depending, of course, on how you connect via WebFTP.
If, however, you connect via http://webftp.dreamhost.com, you will either be placed in your user directory or a domain’s directory depending upon what you enter into the “Initial Directory” field of the Net2ftp sign-in form. IF you leave that blank, you will end up in your user directory, but if you enter a domain’s directory here (for example “/mydomain.tld”) you will be placed into that directory.
Using other FTP clients is much the same as this last scenario, as many clients allow you to define an “initial directory” (though some use different terminology). Doing so will have the same effect of putting you in a domain’s directory while not defining such a setting will place you into your user’s directory, even if you use a domain name as the “server name” for the connection.