Starting point yes, but not ending point. If we weren't supposed to adjust the sliders to get the sound we want they wouldn't be there. The Holy Grail of 'Flat Response' is just as mythical as the other Holy Grail. No amp is flat, nor is any speaker. They all have coloration. EQ is there to be used. That doesn't mean you should go nuts with them, more than 6dB of either boost or cut is generally counterproductive, except for the lowermost slider, which often works best pulled fully down to serve as a rumble filter.
Um... that's why I said starting point.
I'm just saying that it makes things simpler to adjust if you start out with everything flat, as that's the basic sound of the rig. From there, it's relatively simple to decide what to cut and what to boost. It's just a matter of making less work for yourself.
Of course, it helps to know what flat is on your amp first. For mine, flat is lows at about 2, mids at almost 10, and treble at 2. My tone settings are lows at 0, mids at 10, and highs at about 2 or 3. There's also a graphic EQ, but since it puts transistors in the signal chain, I don't use it unless I need more mid boost. My amp's very weird though.
On my old amp, I just started with everything flat and then boosted the mids and treble a bit. On the graphic EQ it was sort of a wobbly "W" shape, though it changed a bit.