Yeah, the rule of thumb still holds, that is, '
cuz would be the right way.
Running a search within Google we see both ways, but as always as you remember, don´t use without the apostrophe sign.https://www.google.com.br/#q='cuz+I
While on the subject, 'cuz
is highly informal, to many people it´s overly so. Thus, if you are writing to a Forum, say, from the UK, don´t ever use it. They are very fond of their language and aren´t happy with outsiders tripping on it like this.
It´s fine with and among friends, anyway it´s advisable to know if they are as informal as you are.
As you can see from the search, it´s very common within music world.