The 3.9L case I'm building in will not have any front IO. And this is okay since I don't really populate all the USB slots but I also realise it won't have a 3.5mm audio jack.
I just got the k49 and I'm loving it. With how small it is, plugging my headphones in at the back is barely an issue. The only tricky thing is actually plugging it in, but the case is so easy to just turn around or move about as needed. Maybe get the case first and then decide?