It will depend on your skill as Ken said. How thick of a piece are you starting with. The problem, as I see it, trying to go to 1/8 all by hammer is that as you approach your desired thickness you are one hanmer mark away from a problem you cannot fix. If you want 1/8 and you forge to 1/8 you have no room to smooth out hammer marks. If you want forge finish steel then maybe it will be OK. If you want machine or hand finish maybe forge thicker then grind to finished dimensions. Just my opinion...Ken and I don't leave hammer marks so its been a while Lol Lol.
You might want to get yourself a flatter if you want to try it by hand. It would be a lot of work but it would prevent making hammer marks on the bar of steel. Blacksmiths Depot and Centaur Forge both carry them and the are instructions on how to forge your own. Remember that a flatter is placed on the red hot steel and struck with a hammer. It is not used to strike the steel itself. They will run a bit more than a good blacksmith's hammer but way less than building a rolling mill.
This is the heart of the problem, if you want to work sheet stock, you need to think more like a tinsmith, coppersmith or silversmith and less like a blacksmith. Can it be done? ... uh yeah, there was a time when it was the only way to do it. How was armor made? I have seen iron scabbards from the La Tene period (around 200 B.C.) made from 1/6" sheet that was obviously entirely hand hammered from a block of steel. As Chris mentioned, it is the grinder, not the hammer, that is the threat. Instead of thinking of the forging as the rough shaping and the grinder as the fine finishing, you need to think of it as hot forging as the rough finishing and cold working with planishing hammers as the fine finishing. If you try to grind huge divots, dents and hammer marks out of thin stock, by the time you get to the bottom of them, you will be undersized or even grind through. Instead you switch to a small, polished, planishing hammer and work the high spots down and the dents out. If you are patient, you will be surprised at how smooth you can make a piece of metal. A silversmith doesn't grind things smooth, he planishes them. Of course the number one thing to remember is - anneal, anneal, and anneal between cold working.
I was envisioning Forgedog doing something like starting with a 1/4" bar of 80CrV2 and trying to reduce it to a thickness of about 1/8" before profiling and grinding in the bevels. If what he has in mind is working with a wide sheet of steel then he is looking at more like what an auto body technician (or an armorer) does and Kevin's advice is right on.