I believe stuff in boxes is worth more. It’s especially evident with prewar stuff, but it’s true even with MTH. Trains ship much better in the original box too. I have proven my theory by looking at completed eBay sales.
The counter argument is this. If you don’t care about 10% value of an MTH box, then it probably isn’t worth the space it takes. If you don’t sell much, then it’s probably not important to you. It’s easier to box stuff up without the original box. Not everyone has space for boxes. I have a crawl space (sealed) full of them. Some aren’t even original, just storage boxes.
George