Over the years I've seen the debate on tensioning or not tensioning a warp during beaming. One view is that beaming with zero tension is 'equal' tension.
Unfortunately while that may be correct, it is not particularly helpful in many instances.
In my observations, beaming with zero tension may work in a narrow range but may very well stop working when that narrow range no longer applies. (And yes, I tried beaming with zero tension - once - and it was a disaster. That warp eventually wound up in the round bin as a sacrifice to the loom goddess...)
Let me give some examples:
Let's say hypothetical weaver has been weaving with 5/2 cotton, mostly short warps - 5 yards and under. And beams with zero tension and has perfectly satisfactory results. Let's say that said hypothetical weaver wants to up their game and decides to use that 5/2 cotton and make a run of tea towels, decides to wind a warp of 15 yards and beams it with zero tension.
This may not work so well.
In my experience, the longer the warp, the more tension it needs during beaming. I have written elsewhere about my experience beaming 14 meters instead of my usual 11 meters and how tension issues cropped up during weaving as the yarn slithered and slid unevenly as I wove. I added four cups of water to each jug I use for weight and voila, no more tension issues with the successive warps. A Swedish friend commented that the guideline in Sweden is that for every additional 10 meters, you need another person to pull tension on the warp while beaming.
In my experience, changing from one yarn to another of a completely different nature may mean beaming with zero tension doesn't work well any more. For instance, changing from that tried and true 5/2 cotton to, say, linen. Or silk. Or a very elastic wool. Or a very slippery yarn like rayon. And suddenly, beaming with zero tension may not look like a very good practice after all.
It can also depend on how much tension is applied during the weaving itself. Something like linen that requires fairly high tension during weaving may mean increasing the amount of tension during beaming. In my experience, the warp needs to be beamed with at least as much tension as will be used during weaving.
So for those weavers who don't use any tension at all during beaming, be prepared for things to change when the parameters of the 'usual' warp change - like warp length or yarn being used.
In terms of where to start? I use about half a gallon of water per jug for my 'usual' warp of 11 meters. I use old bleach bottles because the plastic is a much heavier grade than a water or juice bottle. In the picture above one of the jugs has to be at least 25 years old, the other is newer because the cap failed on the other one I had been using for about the same amount of time.
Currently reading Sunshine and Rhubarb Wine - a memoir of a friend's mother who lived for 105 years.