At what point in a build do you have to have the comp pinned and welded?

Before putting it on a rifle lower.

FPNI

I put it together and the last thing I do is put the MD on, then drill, pin and weld then I stick it on the lower.  It is can be illegal to own a non pined upper if you have a lower configured as a rifle that it can be put on, same as owning a pistol upper without having a lower in a pistol configuration.  Not that I like the laws, but if you have the pieces in your possession, it will be construed as having a non stamped SBR.

ETA, just to add,  I always have a lower that is configured as a pistol lower in the safe.

Usually I'll drop the upper on a pistol lower and test fire the hell out of it to help ensure that there are not any running issues, once satisfied that its good it gets pinned and placed on whatever lower it was destined for.

I don't understand this question, buy am currious as to weather or not I need to do this.  Please explain.

If you have a barrel shorter than 16" with the muzzle device, if it is going to be used on a rifle lower, it will either need to be pinned and welded to make the overall barrel length 16" from the face of the bolt to the end of the barrel with MD, if that can't be done and you still want to use on a lower configured as a rifle, you will need to pay for the tax stamp for the lower which is $200 and takes quite a while to get.  If you don't want to do that, you will need to make sure and have a lower configured as a pistol lower, you can own and use one that is shorter on a pistol configuration without the tax stamp.  I do my own pin and weld on my 14.5" and 14.7" barrels, if you have the right tools, it is not difficult.

When I build one shorter, I just make sure and have a pistol configuration lower to stick it on, then I can use the upper, even if it will eventually become an SBR.

Awesome thank you!