"starter strip"

i've installed five vinyl windows and each came with a "starter strip". i've had problems with each of them staying in the channel while placing the window into the frame.

since the windows fit very well both horizontally and vertically my question is whether the starter strip is really needed?

if by starter strip your refering to sill expander it is installed after the installation which should be in your instructions you have to cut it to height to fill in the space on the bottom of the window and insulate behind it, unlees you ordered the windows to big or did not shim the bottom or you have a flat sill then just run insulation under the window tape it on so when you insurt it it stays in plsace and caulk after you install