The main reason not to unlock multiple levels is when in the sweet' zone. which is (only generally) from level 29 to 65. When a Great Building is in the sweet zone. the owner does not need to add any Forge Points to the GB to cover and lock P1.@ 1.9 and usually the GB can be sniped for a profit just because it is open.
The best way to get blueprints for most Great Buildings is through a 1.9 thread. If your Guild does not have1.9 but uses swap threads. be aware that the players with the greatest daily Forge Point flows will alwasys take the top reward spots. They usually know this, and hope the other players either are unaware, or do not care. The one way to thwart that is to always follow the same person. So you work each others GBs far more than average. Usually snipers love players who use swap threads. Swaps can be of value for very new players..
But best to find a Guild that has an active 1.9. and does not allow swaps. Because half the Guild using swaps means the 1.9 will be slow as cold molasses.
The one main exception to the above is when starting Arc. finding a Guild with a stron Arc program is best. i started my own Arc clubs. and was able to work together doing swaps to get blueprints through the first 28 levels.Even after level15 it is faster and more profitable to use 1.9 threads.and seek P5 positions on high level Arcs for best BP results.
And never do the two for one swap in the Blueprints page. (unless you really do not care about future results, and just want to push it now!)
Before 1.9 came along swap threads were the standard way that everyone used to level GBs. As already said they had their obvious faults if they were guild wide. But if kept to 5 people with GBs at the same level they can help get low level GBs up to a point (level 25 - 30) where they become more cost efficient to put on 1.9 threads. So you can start a 5 person GB club in your guild to help in that way. As that way all 5 get their GBs leveled and also get one of the 5 paying spots on a GB. Also almost all GBs are age specific Zeus fir instance is a Bronze age GB so aiding bronze age buildings help towards getting Zeus BPs. CoA is EMA so aiding EMA buildings help get BPs for it and so on.
The five member swap is also a target for snipers. I found the best way was 3 until the sweet spot with the rest going into 1.9 once 4 is locked. After the sweet spot 3 is getting risky too. During only unlock when your folks are ready to place on 1 and 2