It's a common, and often relevant, question. When you are tightening Building Fasteners, Line Fasteners or Marine Fasteners like square head bolts and Heavy Hex Bolts (or any bolt for that matter) is it better to use the bolt head or tighten using the nut?
Although we could get into torsion and torque, there are a few general rules to remember. The answer to this question depends a lot on the materials being joined together. If the two materials being joined are the same and the bolt is running through holes drilled in the same diameter (for example two pieces of wood being joined together to form a laminated beam or two pieces of overlapping metal being bolted together to form a sheet), it will not matter what you use to tighten the fastener. Tighten from the bolt head or tighten from the nut, either way you will not affect the materials or the fastener strength.
If however, you are bolting together two different materials (aluminum to steel, steel to wood, or even galvanized to non-galvanized steel), it's important to note that the friction coefficient will be different for each. In that case you will need to calculate whether it is a good idea to tighten from the nut face or the head face. Choose the one that demonstrates the lowest friction coefficient. Otherwise you run the risk of increasing the preload or creating such a major difference that bolt breakage is a possibility.
Another commonly seen case is when the bolt hole in each piece of material has a different radius. Generally, in these cases, it's best to tighten the bolt from whichever side has the larger radius. This is because the preload created will be less than if the tighter fitting side was used.
When a washer is used, remember to try and keep the overall diameters of each (bolt head and nut/washer combination) the same, if possible. The closer in size they are, the better the fit.
In short, there are times when it matters whether a square head bolt is tightened using the bolt head or the nut. Depending on the type of material and the diameter of the bolt, it may produce a more solid fit to tighten one way. Generally this is only an issue when you are using torque control and often the difference is a minimal one.
Started my career in the fastener world in 1969 at, Parker Kalon Corp. a NJ based screw manufacturer located in Clifton, NJ working in inventory control, scheduling secondary production and concluding there in purchasing. In 1971 I accepted a sales position at Star Stainless Screw Co., Totowa, NJ working in inside sales and later as an outside salesman, having a successful career at Star I had the desire with a friend to start our own fastener distribution company in 1980 named: Divspec, Kenilworth, NJ. This was a successful adventure but ended in 1985 with me starting Melfast in August 1985 and have stayed competitive and successful to date. Melfast serves the OEM market with approximately 400 accounts nationally.