It's better, but not essential. It is essential (in a sense at least) to have a good idea of whether a bike of a certain size will fit you.

If you ride a few bikes, and find that a certain size seems right, that isn't a bad guide for a first bike.

It helps if you understand the critical measurements or dimensions, and know the numbers (or ranges) that are best for you, and if you know a bit about geometry.

If you can, take a good long test ride -- or better yet several or more. Or maybe borrow some friends' bikes, or go ride with a club or group. There are a lot of helpful people in these groups. From them and from your own experiences while riding, you can learn a lot about what to look for and what is involved, and what works for you. And maybe do some research on the subject of fit -- it's very important for quality of ride.

There are some websites that deal with the subject of fit, and do it fairly well. It's an important topic, and it really helps to learn more about it. It may seem complex at first, but it isn't once you get into it a bit, and get past the initial learning curve.