I had the same questions, so I did some experiments that you can read about in this thread. I was experimenting with pulse & glide and I wanted to know how hard to accelerate during the pulse for maximum FE overall. Here's a summary of the results (see the thread for more details). The MPG numbers were over a 5 mile segment.

This is a very specific test on a particular stretch of roads, so results in other conditions will vary, but I think it shows that there's a 'sweet spot' with regards to the rate of acceleration. Accelerate too hard and you use too much gas during the pulse. Accelerate too slowly and you actually use more gas overall because you spend more time accelerating, and cruise FE is always better than acceleration FE.

Of course, this completely depends on how much time you will spend in the cruise segment after you accelerate. The more time in cruise, the more time you have to 'make up' for the lower FE you got during the acceleration (slow or fast). This experiement was one extreme; there was no cruise, just a glide after the pulse, so the amount of time spent in the pulse was critical to overall FE. It does show, however, that slow acceleration is not always best, especially in city driving where you don't get to spend a lot of time in cruise mode. Think about it this way, if you accelerate so slowly that you never get up to cruising speed before you have to slow down again for the next light/traffic/whatever, you will spend most of your time below 40 MPG (at best) and your tank average will suffer.