Use .psqlrc to set defaults.
For the specific case of \x, newer psql versions (9.3, I think, but it might just be the 9.4 pre-release) can automatically switch to expanded output mode when the rows are too wide to fit on a line. From \?:
\x [on|off|auto] toggle expanded output (currently off)
So I suggest putting \x auto in your .psqlrc, rather than forcing it to on.