If you're using a solid state amp that would be okay. But if you're using a tube amp you still need speaker load. The line out doesn't turn off the output to your cab, its just outputting the signal. I'm not sure if it is post preamp or post poweramp.. I think that might change with the amp manufacturer. On my rivera, the master volume affected the line out volume, not just the gain.
If you're just doing some scratch recordings it should work just fine, Its a good way to get your ideas down. but as far as a useable signal it's not going to get you very far if you actually want it for a recording. A guitar speaker acts as a filter. Its like an equalizer thats always set to the same settings. and its always an improvement on your sound. With the line out you just get the direct signal, technically just the voltage of your signal into whatever you're recording.. so you're not actually getting any kind of aural presence at all. What I always did for recording my guitar in a recording i wanted to actually release was mic the cab like you would at a show, find that sweet spot, its all subjective, there is no wrong way to do it ( well there is but you'll be fine, I'm sure ) and Also output the line out, and another mic a few feet away from the amp to capture some of the room sound as well as letting the sound "breathe" a bit before it gets to the mic. Of course you'll need some sort of multitracker to do this, or an interface to your computer with more than 1 track.
That way you can mix the signals together to find the right sound. You get the "live show " sound from the close mic. then you add in some of the tightness and brightness of the signal with the line out, and the room mic adds some ambience and a bit more presence to the mix. Typically, the line out was by far the lowest of the three in the mix.
But you could also run into phase problems.. which is something I'm not much of an expert on yet. Which is probably why my records don't sound as good as i'd like them to.
in short. don't unplug your cab if your'e running a tube amp. or at least, use a dummy load.