In my opinion, one should always plan it out, otherwise, it's not technically "designing".
That is to say; simply picking out a color scheme and layout that looks cool, and deciding on
whether or not to use rounded corners is not design.
A plan or drawing produced to show the look and function or workings of a building, garment, or
other object before it is built or made.
Decide upon the look and functioning of (a building, garment, or other object), typically by
making a detailed drawing of it.
With art, you can do whatever you want. And one can argue that there is a certain level of art
in web design. To "design", however, literally means to research and plan out the functionality
of something before it is developed.
For example, designing a car requires research into trends in style, feature demand, safety
technology, space required to fit components/people, etc. One doesn't just draw up a cool
looking car and start building it. For websites, you'd research things like color theory, current
trends in style, features the competition include on their website, who the target audience will
be (This is the big one! If you're designing a website for a senior citizen community center,
you would probably use a larger sized text.)