6am Flight out of Rome.. Cab?

First, if it is a 6am flight then it is not a direct flight home but rather a connecting flight to somewhere else in Europe. So an hour and half or even an hour is more than adequate. I would plan on ordering a taxi for pick up at 4am. Should get you to the airport no later than 4.45 and probably closer to 4.30. Get checked in, there will be no lines at that time and have a latte.

I left Rome last month via an early flight to London. My Hotel, which was 3 blocks from the Colosseum, ordered a taxi for me for the next day at 05:30 am. The taxi showed up on time and it was a 30 minute drive to the airport as there was little traffic to speak of that early. It was a flat rate of 48 Euros which was stated right on the side of the taxi. The biggest problem I had was finding out where to check-in for my British Airways flight. There was one airline (foreign) that I had never heard of with A LOT of check-in space. Oops! I suspect the "foreign airline" was really Italian for check-in or something like that. Check the monitors above each check-in station to find the right airline. :)

Great info, thank you both!

I'd try to arrive at the airport 2 hours prior to the flight, especially if checking luggage. Some days airports even like people to be 3 hours early for international flights so play it safe. Definitely a cab, and to my knowledge that fixed rate info is correct. You could also book a private transfer from a service if you prefer that peace of mind. I don't know if taxis still charge from the minute you call them as opposed to the minute you get in (didn't take a standard taxi on my last trip) and I don't think they should with the flat airport rate, but I've seen them try to pull all kinds of things in the middle of the night so just keep that in mind and stay firm with them if you opt for a taxi over a transfer. Of course there are taxi stands all over the center.

There are no intercontinental flights to North America before 9:30am, therefore you are on an intra Europe flight connecting to some European hub (I suspect it's the LH 1849 to MUC but you don't say).
Showing up 3 hours ahead would be a total waste of time because check in positions open only 2 hours before departure for intra Europe flights. They all require that you have completed your check in and have a boarding pass in your hand at least 45 min prior to departure (35 min if business class). Being in line 90 min prior to departure will give you ample cushion. The taxi ride from the city center to the airport is about 30 min at that time of the morning. A bit less if you are in Trastevere.

I'm not traveling for 90 days but I just scheduled my arrival pickup and departure pickup with romecabs.com so that I have it set for pickup. Romecabs will pick you up at your hotel or airbnb, whichever you prefer. The rates are quite reasonable. They recommend pickup three hours before my flight time. I'm getting on an international flight, tho, so that might impact their pickup time. I'm also leaving at 9:30 a.m. instead of 6:30 like you, so there could be work traffic. I see that you're doing a domestic leg before your international flight.

Thanks for the responses; my airline is Veuling and the 6am flight is to London from Fiumicino / Rome I assume.. sounds like I should have a cab get me at 345-4am and arrive around 415-430 and give myself an hour or so before boarding. Thanks!