I know exactly what you mean, I'm at that point with my current car...but at the dragstrip or when I need to perform at my absolute best, a few quick glances at the tach are necessary for a couple reasons.
First, upon an aggressive launch, I monitor the revs as I "feather" the clutch. It is important to keep revs at a safe point before the clutch is let all the way up to avoid wheelspin. If I know that my car will bust the tires loose at above a certain RPM in 1st gear, then I must acheive that point while feathering...and then go full throttle once I know I'm truley hooked-up to the tarmac.
Second, while in the first couple gears, the rpms pile on real fast. To bang the rev limiter or to shift 300 rpm too early could easily result in 5 thousandths of a second or more lost.
I'm not doubting any of your abilities to drive by "feel"...I do the same pretty well too. Maybe I've run one too many bracket races where consistancy = trophy. Or maybe I've lost one too many races by .001 secs. Or maybe I'm just too anal!
Here's a little story so you guys know where I'm coming from:
There was this one time when a friend of mine decided to run his car down the 1320. He made many runs driving the car according to him "the best I know how to"...the results varied by as much as .7 seconds. I asked him to let me give it a try. He says "man I've been driving this for 3 years, you think you can do better than me?" My first run, I smashed his best time by .3 seconds. Why? Before I ran it, I called up another friend to check the best shift points off of this Toyota enthusiast website. I adhered to them stricly, and got a beautiful launch...as a result beat the best magazine times by .2 secs.
It is this level of precision that gives cars with good automatic transmissions (E55) an edge over others in day-to-day situations. Such cars will give 99% of their performance 99% of the time. A stick?...hmm...well that depends on the driver and how masterfully he can use it.