I generally approach playing an 8 string the same as playing a 5 string bass. I try to use the low string sparingly. For example ..you have a track thats written in drop E ...where do you go after that? Surely not lower otherwise it just starts getting f*ckin' stupid. My approach is to say write a track in B with the lower string tuned to either F# or Drop E so I have that extra low to go down to emphasise the br00talz ...if any of that makes sense