first wedding for a friend

taking the photos at a friends wedding soon.
have 10D 28 - 135 IS and 430EX

never used the flash..do i leave it on for outside shots as well in P mode ?
or do i use AV (as well as flash.

inside church quite dark so will P mode be best.

seperate question

if i used a sekonic L358 Lightmeter (other friend) took an incident reading, would the flash used outdoors over expose it?
sorry to be a dummy with basic questions, but would like to get a few good shots if poss.
Many Thanks

Hmmm. If someone asked those questions before taking your wedding photos would you be worried? I would be.

First rule of digital photography: try it and see.

P mode is probably good call. In general using flash outside is good, it reduces shadows under the eyes, and produces nice catchlights in the eyes. Set your FEC to -1 2/3 or so. Inside flash can be ok, though I often use no flash. See rule #1.

I have an FAQ linked from my sig, and this thread might teach you something too. Since I wrote that latter thread i've probably changed just about everything I do, but the FAQ is quite new.

Good luck!