In terms of precision, cooking is a spectrum. There are instances where eyeballing is sufficient — such as deglazing a pan with roughly half a cup of wine — and there are instances where accurate measurements are vital — such as in baking finicky pastry or most recipes from Serious Eats.
Measuring your ingredients by mass (that is, on a kitchen scale) will give you the best, most reproducible results, but if you insist on measuring by volume, you have to make sure you’re using the right cup.
You see, even though dry measuring cups (the scoopy-looking ones) and liquid measuring cups (the usually glass ones with lines on the side) both claim to hold a “cup”, the reality is that a dry measuring cup will hold a bit more. When I filled a dry cup with water, I found it held an average of 10g more water than my liquid measuring cups.
This probably wouldn’t make a big difference in something like spaghetti sauce, but 10g of extra liquid — or 20 or 30, depending on how many cups the recipe calls for — could really mess you up in baking, or ice cream, or candy making.
Also, dry measuring cups just aren’t designed to measure wet ingredients, and vice versa.
The correct way to measure a liquid is to place your measuring glass on a flat surface, pour the liquid in just below the line, then bend down to bring your eyes level with the lines of demarcation before slowly adding more liquid until the bottom of the meniscus (the curved line that forms on top of the liquid due to surface tension) is level with the line.
You simply cannot do this with a dry measuring cup. Not only are there no lines, but you can’t see the meniscus.
Conversely, the ol’ scoop-and-scrape dry measuring method won’t work in a wet measuring cup, as the “cup” line is below the lip of the glass.
If you’re working with a really finicky recipe, a scale is always your best friend, but if you don’t have a scale, or find yourself in a kitchen without one, make sure you are using the right “cup” for the job. It could be a matter of cake or no cake.