What kind of planer are you using? Hand held such as Mafell and Hema can manage the task but the process can involve several steps and many passes. The ordinary small woodworking shop planer will not allow typical timbers through the mouth nor is there sufficient in/out feed support. A heavy timber can tip a planer.
So help by presenting details on the machine and the timber sizes planned.
Sorry, it's a 15" stationary, and like I said, 6x6, 6x8, usually 12' and under, but sometimes longer. I usually have two guys surfacing, one at the outfeed and we haven't tipped the planer yet :-)
Wood in general strongly resists homogenization...Taking timbers (or wood in general) and making it "square" (or trying to...I have seen endless "reinventing of wheels" and/or trying to force a natural material like wood to conform to an inert material like plastic, or metal...but even these expand and contract, as well as, "move." ...The effort (comparatively) to making wooden members "perfectly square" is not worth the work to achieve it...in most cases especially in timber framing...
Plane the wood for esthetics then employ layout methods that build to the "ideal." This is why "scribe rule" and "line rule" evolved millenia ago and are still in practice today as the dominant forms of layout in timber framing...from a globle perspective of the craft still practiced...
I have used a 13" for timber stock as large as 150mmx200mm (~6"x8")...So a larger 15" planer would more than handle these in softwood timbers, and with a bit more challenge with heavier hardwoods. Sounds like you are just cleaning them up, and not actually "Jointing" them into perfectly square stock...Which hand held traditional manual planes and/or power hand planners would do just fine for such cleanup work...
If wanting to square timbers that is the job of "a jointer"...and...much harder to achieve with things larger than 150mm x 150mm...
There is (in Japan) a smaller "handle held" squaring jointer/planer for "Neda" "Nuki" and other such members in Japanese framing that is a two sided planer that forms a perfectly fixed 90 degree angle...It still would be a challenge over proper layout methods comparatively...
