I might get flamed for this, but in my opinion, letting a friend of the opposite sex sleep in your bed platonically is letting them take liberties with your masculinity (or femininity, as the case may be). If she's drunk and can't afford a cab, or if her car broke down, or if the floor is covered in broken glass, then sure, yeah, okay. But the bed is a special domain. Your personal immediate space is a special domain.
The world is full of people who will invade that space in order to get you attached to them, for whatever reasons, but precious few of them having to do with honest, sexual affection. The world is full of emotionally selfish people who will jump at the opportunity to inhabit your personal space because they know you want them and love the attention, or because they are into you but are too immature to go about things the right way.
If she beats around the bush in getting around to telling you she likes you, 9 times out of 10 she will beat around the bush about other relationship issues, including the kind that should have been getting immediate attention.
Your personal space is ideally the last step, or near the end of the chain leading to something fulfilling, whether short-term or long-term. It is not a tool.
But once that last step is taken, for Pete's sake, don't wait until
then to ask for permission!
It's already been given.