Personally, I would not count on gravity alone to clear the line. Horizontal PEX runs are notorious for having low spots and these low spots will hold water. On my 8289ws, I have a low spot in the BTF line where the line crosses over the top of my pass through storage.
Blow the line out and/or insert antifreeze.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)