Just to add I have traveled at least 6-7 times with family in last 1.5 years that includes both ways but honestly Alhamdulliah not even a single time I was being stop by anyone. I strongly second Moderator input as I myself never faced such issues.
Whereas on one round trip due to some reason I don't have vehicle documents with me at all. I only carry Licence with me. Nobody checked anything on both side.
Vehicles on which travelling was done
1- Pajero 88 Islamabad No (Lahore to Karachi then Gawadar and back to Lahore. (No such issues...)
2- Indus Corolla KHI No Lahore to Karachi and Back to Lahore
3- Corolla Altis KHI No Lahore to Karachi and Back.
With the grace of Allah I have very pleasant experience so far on this entire route.