You can keep it closed, the external monitor will (temporarily) become your main monitor, you just have to have a keyboard, mouse, screen, and power connected to do it. It will go to sleep when you close the screen, once it does, just tap a key on the keyboard the wake it up. The MBP is designed to operate fine in this mode, but many people believe it to be bad for the screen. If your fans begin to run a lot, simply open MBP to allow more air to cool the system. This will not automatically reactivate the built-in screen, when you want to do that, open system preferences for display then click on "detect display".
A riser is a good idea under any and all circumstanced. I have an iCurve at home, and use three post-it notes pads at work, just enought to allow for cirulation (but I use the screen at home, not at work).
TEG