There's no such thing as 'rotate speed' with a SEP. When it's ready to fly, it will.
Bad airmanship to hold it on the ground after it wants to fly. Also shows poor mechanical sympathy on the part of the 'driver' (think of those forces on the noseleg it was never deigned to take!). Nosewheel should be unloaded early in the t/o run (and while taxying)... and actually held off ASAP on a rough or muddy field.
Sometimes (muddy field) one drags it into the air before it's ready to fly to get the wheels out of the gloop, however, and accelerate just above the surface in ground effect before climbing away (know your aeroplane before doing this).
A party trick in the Yak was to get the gear up ASAP but hold the aeroplane down just above the runway, accelerating all the time and requiring more and more forward stick to hold it down. When the trees at the upwind end were starting to fill the windscreen, ease back and up she went like a rocket, almost vertical. For about 350 feet! Then a very careful push-over to level flight! Why? Because it was fun and because one could!