I almost always use faction ammo. +15% dps over t1 is pretty much always worth it, pve and pvp.
T2 ammo is useful for the extra damage or range when you can control the engagement.
lets use blasters as an example, void is high damage, but low range and tracking. So if you have dual webs you can probably use it vs ships the same size as you, or in most cases when fighting larger ships. If you can’t control range you probably don’t want to use void as they will either go fast enough you can’t hit them, or they will pull range so you can’t hit them.
vs other ships the same size or smaller if you don’t have speed/range control you probably want to use the faction ammo for the extra tracking.
then if other ships have the range control and want to try to kite you have null ammo to try and hit them at longer ranges.
Now lets switch to beam lasers.
gleam is useful for short range high damage gets a tracking bonus but a range penalty, Does just a little more damage than faction mulitfrequency. Pretty much always worth carrying, but I wouldn’t use it as a default.
Faction ammo for stuff in the middle, lots of options for the various ranges
aurora for extra long ranges, but has low tracking.
then there are missiles, fury/rage for maxing damage and hitting big stuff, precision for hitting small stuff, and javelin for hitting at longer than usual ranges, or faction for decent all around performance. Most NPCs are big and fat for their class so most pve ships can get away with using fury/rage for most targets, although you still probably want some of the other ammo for some targets.