It is unsafe to walk too close to the front of a fixed wing power plane. There may be a very short pilot whispering "clear prop!"
It is unsafe to walk too close to the rear of a rotary helicopter or a gyrocopter which is already running...and I used to think it was unsafe to walk anywhere near a heli without keeping an extremely low posture....still true or not?
It is perfectly safe to walk all around a glider unless it is attached to some sort of launching line. It is dangerous if you are standing on or near the winch line or the aerotow rope. (once in my former club we nearly launched a chap who caught a foot in a loop) If both wings are level, a glider may be about to be launched. If one wing is on the ground, probably not, though there may be somebody groveling under the glider attempting to attach a winch line to the belly hook.