If you can take some of the pressure off your feet by committing nicely to the harness and creating a lot of mast foot pressure, the board will become less sensitive to your feet position and movement. However, as Roger pointed out, if you're underpowered due to too small a sail, this becomes difficult and, as I also previously mentioned, being underpowered you shouldn't even bother going for the straps.
The problem with "sitting in the harness" is that you're not making the most of your sail. Your sail is most powerful when it is upright, and if you're pulling it to an angle by sitting in your seat harness, whilst underpowered, getting up to blasting speed to get in the straps is going to be difficult.
However, being able to sit in the harness is a very good technique for being completely over powered (at which point, I find, a seat harness to be a life saver). For me, I get overpowered quite quickly at 55kgs. When I am overpowered, as soon as I come out of the water from a waterstart I'm straight in the harness and have almost no weight on my feet, just enough to drive the board forward. In this sort of situation, knowing how to commit all your weight to the rig via sitting in the harness is essential.
The more comfortable you get with sailing hooked in and making harness use second nature, the easier it's going to be to consistently get into the footstraps and get the board flying!
JP Super-X 106l, North F8 Ram: 5.8m,