In the book 'Colloquial Gaelic" they claim that a-staigh should never be used in this context as a-steach implies inward movement and a-staigh implies being inside.
...and then I saw this quote from Dewelly that a-staigh:
is employed both of motion and rest in Perthshire, a-steach not being used there at all.
I am a rank beginner and therefore easily confused and lacking confidence to just plump for one version or t'other. So can I use either? Does it matter?