I checked my old Nike+ foot pod (in the midsole of a Nike shoe) against my Garmin once. They were almost spot-on for a certified 5k route I train on. I don't think a holder on your laces will make a difference to the sensor's accuracy.
And Nathan makes a foot pod holder that seemed pretty sturdy and secure. Never tried it, though.
“Everything you need is already inside.” -- Bill Bowerman
Not quite right
Funny topic atm . I happen to have a pair of Nike shoes and a holder for my none Nike shoes and used both. To be honest I have noticed a little difference between the two when using them on the treadmill. Outside I use my GPS and sensor.