webmail.sample.com doesn’t exist by default. It’s not anything. That’s why it would be fine to use it if that’s what you want to do. It doesn’t conflict with anything.
But you probably would be better off with
servername.sample.com/webmail in the long run.
It’s not a huge big deal, but if you wanted to do certain things in .htaccess, using a sub would add another line or two of code. Because it will work exactly the same either way, I just trail it as a subdirectory to the root of the hostname:
Maybe to clarify, Roundcube doesn’t care about the directory name it’s installed in. It can be
/gertrude. If your users will be coming from a cPanel background, then
/webmail would be a good way to go, just for consistency’s sake. Otherwise, the directory name doesn’t matter.
It also doesn’t matter if you make it a sub in terms of how well it will work. It’s just slightly easier to trail it.