Is it ok to leave my laptop on?

It'll probably go into sleep mode anyway.

The environmentally correct answer is not to leave it on but, if you must, have a look at the power save options (perhaps in Windows or in the BIOS) to make sure you have these set suitably.

Sure, you can. Make sure you check on it's temperatures and keep the fans and heatsinks clean. If you have the ability to put it to sleep, though, it is the better option.

The only thing wrong with keeping it always on is that dust collects, otherwise computers are happiest being left on. However for power reasons I have my laptop set to turn the monitor off after 15 min and go to sleep after 2 hours.

If you plan to leave it on for an extended period of time, you should make sure that your fan intake vents have enough room to get some clean air. You shouldn't leave it flat on a desk at a wall corner, but try to put it in the center of your desk and elevate the back (if the fans are back there). That way, you get some nice clean air running through your computer to keep temperatures down.