Lame excuse. "I'm kinda new" won't cut it with the accident investigation team. It also won't cut it with the ghost-riding FAA inspector who is sitting in a window seat over your wing watching the snow accumulate on control surfaces prior to takeoff.

If you think deice is required tell the Captain, "We'll need to be de-iced." DON"T ASK HIM, TELL HIM. Make an affirmative statement of the course of action YOU would take if YOU were the PIC. Any worthwhile Captain will listen to your opinion and make the correct decision. If he doesn't agree with you he should tell you why he is making the decision to not deice.

I'm not suggesting every time an FO thinks deice is required you will get it. It's ultimately the Captain's decision. If you feel strongly and the Captain still says "No Deice", then show him the frost/snow on the aircraft, quote the FAR that states "no aircraft shall takeoff with snow, ice, .....". It might even get to the point of telling the Captain you will be filing a NASA report after landing due to a perceived risk to safety or you may even need to refuse to fly the leg if it's really bad. I've never had it get that extreme in a cockpit. Most times the Captain will take the most conservative approach. The ones to watch out for are those guys who think they can personally save the company with their superior money saving efforts.

If you need deicing, get deiced. Your passengers are depending on your judgement.