Having a Kassadin on Your Team

If you have a on your team and they are ahead in kills/items early what should the rest of the team be doing to capitalize on that? I sometimes have Kassa's on my team, or I am playing as him and we are sometimes fed early, i feel like the proper thing for us to be doing in the game is to be taking objectives while he is ulting around lanes destroying enemy carrys.

Now I could be 100% wrong on this and maybe the team should be grouping together once kassadin is fed. This is why I am wondering what the proper process is if a kassadin is fed from the 5-15 minute mark with either none, or one to two turrets down in any given lane.

To sum it all up, if there is a kassadin on my team who is fed, I will ask him/her for a quick gank on my lane to take a turret, where as I find other people thinking this kassadin is supposed to be taking objectives and killing everyone by himself. Sometimes I notice very little being done while I play kassadin alsol. Meaning that while I am around securing kills so teamates who havnt taken either tier 1 tier 2 turrets can take them, they are instead themselves randomly spread around the map. I teamate kassadins, or myself to be blamed later game for not helping with objectives enough while we have been out there allowing our team to safely take them.