On jet types with Autothrottle, for example the B744, at 65 knots the autothrottle mode changes from THR REF (thrust reference limit - determined by ambient temp, pressure and any derate/assumed temperature thrust reduction) to HOLD (A/T disconnected) to prevent any unwanted thrust lever movement caused by an autothrottle malfunction. So if thrust isn't set by 65 knots the throttles will stay wherever they are. THR REF is re-engaged when VNAV engages which is usually 400 feet above runway elevation.
Also you don't really want to be diddling with thrust levers when your belting down the runway at a great rate of knots, so you're better off setting thrust early and concentrating on keeping the beast on the runway!
EPR will usually increase on the takeoff roll if left alone as the power a turbine engine produces is a product of the temperature, pressure and velocity of the air entering it. I've never really noticed an increase in a jet(RR), but it is certainly noticeable in a turboprop engine such as the PT6.
