I think it's should be based on a foundation of a tried and true but changed at least 51% for it to be "yours". It's like music - it's been around forever but the notes can be rearranged is so many different ways that a new rif makes a new song.
If you start with a "tried and true" recipe and change it 51% - you won't even be brewing the same style anymore. It also depends a lot on the style - personally, if you are brewing something "to style" I don't think there are any real "original" recipes left.
I mean, if you are going to brew a pilsner....... there just aren't any significant options. 90% pilsner malt probably (or more), noble hops, czech or german lager yeast, ferment around 50 (give or take), low mineral water...... What are you going to change and still brew a pilsner.
I ALWAYS start with a "tried and true" recipe (brewing classic styles, NHC winners, kit recipe, recipe from a known source, etc.) I brew as close as I can to "as is" and then from there, I may start to tweak it. At best, my recipes get to the point where I would say "based on." I don't get caught up in "this is my recipe" because there is a near certainty that there are really no truly "unique" recipes out there unless you just brew something ridiculous.
For instance "my" british bitter started with Northern Brewers Innkeeper - but, I adjust the amt. of grain for my efficiency, I used maris otter instead of golden promise, I used half pound of corn sugar instead of a pound, I add a half pound of wheat, all EKG on the hop schedule and I use 1469 yeast...... But, it was all "based on" that recipe kit. Same with all my beers probably. Does that make it "mine?" On the one hand, I changed quite a bit, but, on the other hand...... not really.
The bottom line, in my opinion, is if you take a tried and true recipe, or a recipe for a style and change it a lot - it is highly likely the only thing you are going to do is screw it up.
I think recipe's are a dime a dozen. The thing that makes your beer "yours" is your process.
*Edit - That said, if you use someone's recipe, or tweak it, I think it is right to give credit where credit is due if you have an opportunity.