Do you guys use the guidelines when making a new recipe or do you more-so use your knowledge of past beers?
It all depends on what I'm going for. If I'm trying to make a beer for a competition, I will usually try to find a few sources with recipes and tweak and combine to make my take on it in conjunction with the style guidelines.
Sometimes, I'm making a beer that defies styles, so I have to get a little more creative. For example, I just came up with a recipe for a blonde king cake stout for our club's Iron Brewer competition this past weekend. I had to take several different recipes I've used in the past and guesstimate modifications for spices and flavors with a different base.
In some cases, there's no substitute for experience and brewing a lot to know how various ingredients will come out on their own or together. Come up with a base for something (like an IPA) where you can experiment with a single variable (like hops) and learn to detect the differences.