It is true that due to the laws of physics and inertia and all that sort of stuff, that spinning a liquid will make a parabola.
But In practice it is not as easy as it sounds. As there are 1001 things to get right to make it work.
I agree with Mike Rolon that there is no substitute for gaining optical excellence other than "walking round the barrel" but from my experience it is possible to have a 50/50 solution.
First of all though, here are three of the main problems:
The first main problem with spinning the molten glass is contamination from the enviroment. ie Dirt set on the surface trapped by the now cooled glass and a permenant blemish on my otherwise not too bad mirror. The furnace must be clean from all dust and other grime. Not so easy when you are using heat bricks and concrete.
The second main problem that I found was that as the glass was cooling whilst spinning due to uneven temperature areas within the furnace and other thermal currents created by bad furnace design, the inevitable happened...
The cooling was uneven and the mirror ended up miss shaped - although only noticable through testing. This effect also occured when the spinning table used was not perfectly flat. You may think, as I did, that these problems are not relevent due to the fact that the mirror is spinning but the speeds involved are not sufficiant to overcome.
Smaller mirrors (6"ish) were obviously far less effected by these problems than larger mirrors (10"+).
The third problem that I found was changes in motor speed occured which resulted in ruin! These changes in speed came from an unstable VFO which was soon replaced by a stepper motor, driven from a crystal osc. This system has proved to work well.
Now, about the 50/50 bit.
Once a reasonable casting has been made it should be tested extensively with the foucault test and then the final figuring and polishing should be carried out by hand untill it is to the required finish.
Smaller mirrors can work quite successfully with only some minor touching up to do, but as with everything to do with mirror making, the bigger you get the harder it gets and the more you need to do to get a good mirror.
I have had a moderate success rate with spinning molten glass with several small mirrors made to practice on and one or two well usable larger mirrors now in service.
I do not belive that spin casting can replace grinding by hand but if done well and a good casting is made then it can beat alot of walking in circles "around the barrel".
But a warning:
Casting your own glass is not easy it has cost me a lot of time, burnt fingures, shares in the local gas company, nearly a devorce, a burnt driveway, a big pile of unusable contaminated glass, headaches, backaches, wallet aches not to mention brain ache!! But all this aside, it has been quite good fun building furnaces and developing the skills and tools needed to melt, cast, spin and figure my own mirrors. And now I have the skills and knowledge under my belt it is much easier than when I first started. It is a very rewarding hobbie and my advice to anyone who is interested in it is give it a go, just try it, you never know, you might be more successful than you think. Just start on a 4" or 6" to practice on and then the sky is your limit!!!
Sorry to ramble,