gnow
We always try to nominate a Vtoss higher than the minimum for airfield takeoff (even if it means a backtrack) since 100'/min is not a very good rate of climb, especially if there happens to be a downdraft of 200'/min!
We then beep the Vtoss up to that nominated speed for various reasons - for a clear airfield takeoff so that it acts as a reminder to both pilots what the nominated Vtoss is, and for onshore helipad takeoff so that the autopilot knows what speed to go for if we go OEI before Vtoss.
But then it was me who asked EC for the ability to beep up Vtoss (it was not a feature of the original software version we had with our first deliveries) so maybe I am biased!
To comply with the Cat A procedure, you should not change the Vtoss in mid-stream from that which you based your takeoff decision point on. If you do, whilst it might work out OK, you are effectively making up your own procedure that has not been approved by any Authority. You would certainly invalidate any obstacle clearance calculations that you might have had to do since the graphs presume that the stated profile is followed.
HC