Look molasses is one of the most mythical, debated, etc- growing additives. You look around and people will tell you one by one it does everything- then look around some more and you will see other people saying it won't do those things.
Most people talking about it don't know what they are talking about
It's best use is to provide carbohydrates to mirobial soil life. It's for gardeners who feed the soil not the plant.
To help you understand this concept, let me tell you about Aerated Compost Tea's. What you do with that is get compost or better yet earth worm castings, some kelp and molasses and you bubble that with air stones for 1 or 2 days. The compost provides the microbes and then the molasses is used to feed them. In the aerated conditions the microbes numbers increase and in the end you use that brew in inoculate the soil with microscopic life- which will then go on to create their own food chain which will create fertilizer for the plant. The molasses is not added for K, nor trace elements. The sugar is not expected to make it to the plants- it's presumed to be largely consumed by the microbes (and equally assumed that without it, the microbes would of starved).
So... back to soil... it's still used for the same thing. It has K and trace elements but not in any quantity that is especially useful- and these are things that can be more easily sourced elsewhere. Banana skin for example is a way better source of K. Their is debate whether the sugar is taken in to the plant, but if it's such a debate, that in itself shows the effect can't be too massive or it would be obvious. For example; no one is debating whether kelp works or not. Add to this the fact that tons of organic growers get fantastic results without molasses use (as a sugar source) so yeah...
So bottom line is adding it at any time couldn't hurt- earlier the better since it's for microbes primarily and you want them there before you even planted your seedling in the soil. But having said that, just adding microbe food isn't nearly as good as using the mollasas to brew up microbes in a bucket and pouring that into your soil.