There isn't really any "should" when it comes to webcomics and writing. As others have said, it's all gonna be about finding what works for you! Some people find that writing every detail makes them totally lose interest, or that writing every detail is a waste of time because things will inevitably change along the way. Some people find that inventing as they go makes them frustrated or stressed, and that a lack of planning makes the story come together clumsily.
For some folks, it's gonna be something in between!
For me, I find leaving some gaps to be filled in later helps me (a) not spend forever on planning and (b) be able to change things as I go. If I start out the story just knowing "In chapter 6, they meet the wizard," and as I'm telling the story I realise that there are a lot of things in the current chapters that would probably add some conflict in chapter 6, I have room to explore how those things will affect and change the wizard-meeting. If I had every detail written out in advance, it would require a lot more re-writing every time I thought of a better way to do something.
Since this is my first story comic, revisions like that happen a lot!
But when I don't plan ahead at all, or only in the loosest of terms, I find I start running into problems with story elements that take time to set up. If I know "okay, when we meet the wizard in chapter 6, we have to understand how magic works by the time we get there," then I can start finding ways to show how magic works between now and chapter 6, and make sure that my exposition is smooth and natural. If I don't plan ahead for that stuff and figure I'll just put things in whenever they feel natural, then I find myself going, "okay, next chapter they meet the wizard and... rats, I gotta explain all this magic stuff" and it's much harder to fit it in without a big infodump.
And nothing says you have to stick with the same method if it ain't workin for you anymore. If you start out not planning very much, and find it's stressful, you can pause, sit down and plan the next chapter, and see if that goes better. Or, if not planning is working just fine, but next chapter is very complicated and you feel like you'll do better if that one has more planning -- that's fine too! It doesn't hurt to try stuff. : )