Hello Denny,
FAR 91.159 has a couple of key words in it that would apply to gliders. The first is “or while turning.” Gliders, at least here in the midwest, spend much of the time turning whether thermaling or searching for thermals. The second key words are “in level cruising flight.” Gliders, are always either descending or climbing. There may be an exception if in ridge lift where you might be able to maintain a constant altitude, as the same goes with wave lift. Cloud streets may also provide level flight.
This is why glider pilots must constantly be looking out for other aircraft. The rule helps glider pilots give them an idea of where oncoming aircraft might be coming from and at what altitudes.
Example; you are flying your glider in a westerly direction and maintaining an altitude between 4500 and 7000 feet MSL (lift band). Oncoming VFR powered aircraft traveling between 0 and 179 degrees magnetic should be at 3500 ft, 5500 or 7500 ft . IFR traffic should be at 5000 ft or 7000 ft. The glider pilot should be very aware and searching the sky for traffic while transiting through those key altitudes.
§ 91.159 VFR cruising altitude or flight level.
Except while holding in a holding pattern of 2 minutes or less, or while turning, each person operating an aircraft under VFR in level cruising flight more than 3,000 feet above the surface shall maintain the appropriate altitude or flight level prescribed below, unless otherwise authorized by ATC:
(a) When operating below 18,000 feet MSL and –
(1) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude + 500 feet (such as 3,500, 5,500, or 7,500); or
(2) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude + 500 feet (such as 4,500, 6,500, or 8,500).
(b) When operating above 18,000 feet MSL, maintain the altitude or flight level assigned by ATC.