I always warm-up in a 48X16(81") and i always put it back on at the end of my session because i will always be doing rollers in the same gear the next day.
workouts are done in 1 of 3 different gears, for my entire days workout. My Kilo Race gear or 2" below it or 2" above it.
the theory is that you want to develop your ability to put out power at a range of cadences. So varying the gear size a reasonable amount, that allows your normal speeds and power. you don't want your over or under gear to be so extreme as to affect the quality of the workout. If you can't replicate your normal times or speeds- you are sacrificing quality.
All my leg speed work comes from rollers. I think doing leg speed on the rollers is far superior. First off you aren't sacrificing valuable track training time with form workouts. Second- the entirely Active nature of rollers means that 100% of a high RPM effort is being powered. During big under geared track workouts there is a lot of Passive time- where the legs are just going around with the pedals- which is less effective in improving pedal mechanics. Just like riding a fixie down a big hill at high RPM's, while your feet just spin along for the ride, is totally ineffective training.
nothing but anecdotal evidence on my Leg-Speed theories- but i do feel my training has made me a less gear dependent rider. I can replicate efforts- in flying 200's or race situations through a big range of gear sizes. I am not a Cadence Limited rider..