What Jimmy said is so true, though with most graphics engines (it's important to distinguish it from a game engine), you'll want to modify some code to suit your game, sometimes you'll have to fix a broken feature. Beside that having knowledge of OpenGL will mean you have understanding of how 3D graphics programming works, which is essential.
At the end of the day, which ever engine you choose, the engine is "just" a library, being able to work with it is important if your company is using it, but, i'd say, it's alot more important to have an understanding of the underlying technology, especially that you can do learn it now, when you have time and no deadlines. Knowledge of OpenGL will let you branch out more easily, while the solutions of one engine designer won't always translate to other engines.
It all depends what you want to be doing, it may be that you just want a finished game you can show somebody. Coding in OpenGL will mean that you won't see a game in a long time, it's important to keep that in mind. It doesn't help, that when you create your own engine, if you want tools for it, you have to code them yourself.
There isn't a definitive answer, i'd say, if you're a beginner or intermediate and you want to learn an engine (or an API) while making your first, or second game, don't dive in to OpenGL, simply because you don't know what's ahead of you, there's ALOT more to a game than just graphics. It's better for your morale if you try to bite what you can chew. Finishing a project is alot more important than a list of features and unfinished code.