Ok ive heard this rumour from a friend, he says he heard that putting dimes at the bottom of your valves bottoms, inside your valve will give you better slotting. Is this true. He says it acts as a heavy valve cap.

No, your friend doesnt know very much about playing trumpet and shouldn`t be giving advice for it. Slotting is something your lip muscles learn from dilligent practice at creating sustaine, constant-pitched notes in tune with one another. The wieght of your horn can effect this, but only when properly proportioned (i.e; Monette Trumpets), but three dimes will only serve to rattle about and possibly cover the air holes that allow your valves to move.