Is it safe to add language strings?

Following a suggestion by, I think, @ForeverPontiac both here and in response to a previous request from me in the preview forum, I'm going to attempt to have a language-specific logo on my forum by creating a language string to link to different images. Practicing with an existing language string has worked, so I'm willing to give it a go.

I was going to create a new language string using phpMyAdmin but now I'm no longer sure. Isn't there potentially the problem that a UID that I might give it could clash with one that comes out with a future update? I can see that word_id is of the type int(10), so it doesn't seem to me that I can cheat by using a decimal point ... or could I?

Is it safe for me to add language strings this way? Or is there some other mechanism within the ACP?