Everytime you turn it on ideally. If the dials on the monitor are moved, the picture on the monitor may not be what you're actually getting on tape so its a good idea to check it everytime you take the monitor out.
Might want to check out Serious Magic's DV Rack. It's a software for your laptop that includes a field monitor, hard disk recorder, teleprompter and a bunch of other stuff. Really useful and excellent. And it's much easier to shlep around a laptop than the average CRT field monitor, IMHO
I know that my field monitor, has been dropped more than once. It's cracked on the outer case, but it's still kicking. (I have since upgraded to an LCD though.)
Be careful, most consumer LCD's arent capeable of displaying the proper colors, and their contrast ratios suck! So check your zebras and switch to color bars often to give yourself a frame of refrence.