I've always thought of sweet berries as relatively arbitrary items. Sure, you can use them as a food source and for trading. But pretty much everything else in the game is a better option. The only unique use for the berries is to breed foxes, but for that purpose you wouldn't really need a berry farm.
It's much more economical to find some cows and make a cow farm, and/or find a taiga village and get potatoes. A baked potato farm is my personal recommendation, especially in the early game, because it's a pretty good food source and very easy to farm. As far as when to make the farm - up to you! Having a farm is a great resource as the goal is to provide yourself with a supply of the item which you are farming. So, if that item is in significant need, then that's a good sign it is time to build the farm (for example, I recently realized that I needed practically endless supplies of scaffolding for builds - so I built a large bamboo farm that now provides this). Personally, I don't choose to rush build every farm as fast as possible. Rather, I focus on what I need at the time, then build accordingly (sometimes there are exceptions, because after all - I am a completionist). In fact, I spent weeks playing in my main world before building even my very first farm (which was a wheat farm). It's all up to whatever you want to do!
Hope this helps!