Whilst not wanting to sound overly dramatic, I would be rather concerned if my colleague started arming approach at FL350! If the question is to find out if the thing will arm above 8000 feet, then fine, arm it passing FL100 or so, where I'm pretty sure I can handle most AP-malfunctionings without raising my bloodpressure. But to go experimenting with mode selections in cruise where your options are somewhat limited should anything go wrong, does not make me feel very comfortable.
In these electronic jets, little things sometimes have big consequences! So yes jtr, I agree that the APPR pb is not guarded. But do you want to be the one to find out there is some undiscovered programming glitch? (like the guys who found out that in some cases after arming approach, the aircraft will not pitch up during the subsequent go around, in stead heading steadily towards the ground at increasing speed)