Is there any reason NOT to use all my reinforcements?

[h4x2tehm4x]

Is there any reason not to just immediately call in all my available reinforcements at the start of a mission? Will this cost me anything?

[MeePok]

the less reinforcements you call, the better your game score. www.gamefaqs.com has some guides regarding ranks and medals and such

[Angelo]

some missions will grant you a medal if you do not use them
depending on what difficulty you are playing on you may wish to wait until later to call reinforcements to use them to protect escort ships - usually most of your AI will die before long on the harder difficulties