|Dorothy Lindman Venture-Lieutenant, Washington—Seattle aka Gwen Smith|
It is also going to vary from character to character.
I have a diplomancer Holy Tactician paladin who gives out a ton of buffs to allies who follow her advice. She makes a great leader both in social situations and in combat.
I have an insufferable know-it-all Mindchemist with +18 in all Knowledge skills (at level 4) but -3 on all charisma checks: she makes a decent leader in anything but social situations.
I have a somewhat ditsy Zen Archer who has been known to wander off in the middle of briefing sessions to pet another player's animal companion. If anyone tried to make her the leader, she would laugh in their face.
Probably the best solution is to work it out in character. After the briefing, talk through the roles each character can play and decide who is going to be the spokesman/face, the tactician/combat leader, the scout, the science officer, etc.
You should be able to get a pretty decent feel from the party (in and out of character) for how rigid the "hierarchy" should be for each particular group. Adjust accordingly.
And if you want to have a character be a leader, build one. And during introductions, set your character up to the be the leader. You'll find out very quickly whether the rest of the group will respect the character, humor the character, or ignore the character.
For example, one player in our area runs a high-born noblewoman with lots of follower vanities as an entourage. The player tells the table, out of character, that the character believes she's in charge, and explains the various mechanical benefits of going along with what she says. Then in character, she character explicitly introduces herself as as the party leader, and describes what she expects from the group and how she can help them. Other players can then choose whether or not to follow along. But because they were pre-warned out of character and because the player in question doesn't take offense at the party's decision (even if the character might!), I've never seen it cause a problem.