as long as you're not maxing out the CPU and GPU, it should be fine. I've used a 60W charger with my 15" MBP (85W charger) with no problems, but I was only doing light browsing.
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_3 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8J2 Safari/6533.18.5)
45 watt might run the computer but it will be using the battery to compensate for the loss, but not recommended, would you run a car with a half ass alternator on 11 volts?
What about charging the MacBook while it is off with a 45W adapter? That should be ok though, right?