the closer your dot is from your eye, the easier it is to pick up quickly.
if your shouldering technique is perfect - but who's is? - it shouldn't matter: your dot would be immediate on target.
In the real world, we don't always have that perfect line of eye-dot-target. With the dot up close, you have more room for error in misalignement and still see the dot, and quickly correct your aim.
With the dot further away from your eye, you have a fair chance of not seeing your dot - due to misalignement - and you have to search your dot first, before you can even start aiming.

I used to see a lot of (secundary) dots on the handguard of rifles in IPSC, but now most are placed on a angled mount on the receiver.
Most shooters do seem to find this the faster solution.
This way you also have less chance of damaging your dot when engaging baricades.