Well both. Anything not thc or other cannabinoids is taking up space that could have been thc etc, and can impart a planty bitter taste to edibles. Making it obvious they are a bit special.
Im not saying you cant do it that way, but its not the absolute best. Its still relatively effective. But if your looking for the most potent and most like the herb it came from in regards to effect you really should dissolve decarbed hash in melted coconut oil.
As to the actual extraction, as long as the water/oil is warm enough to melt trichome heads, not much will be left behind realistically. The fats arent exactly an aggresive solvent but 30 minutes should suffice
You may not need to know exactly the mg in your oil, but knowing if you should add more plant or possibly hae too much eases the mind