To focus fire, or not to focus fire?

That is a situational question.  Sometimes you want to override the automated target selection for some of your ships.  Other times there is a capital ship attempting to flee, and you absolutely want to kill it before it can phase jump away.  Other times there is a particular support ship or key orbital structure that you want to remove first.  Shield mitigation might matter too. 

Normally, yes, you should focus fire.  Of course, it always depends on the situation, what units you have, and what you're doing with 'em.

There are some situations I always focus fire. Kill the enemy siege and anti-module (torpedos). If it is a fully upgraded planet, dont let it fall. Starbases can go pretty quick if the anti-modules are left unchecked. Protect your expensive investments.

Carriers are deadly, constantly pumping out strikecraft and the repair ships can keep a cap ship alive indefinatly. Distroying even a few of these can help greatly.

The enemy fleet might have multiple weekend ships. Hold SHIFT and select these in the order you want them distroyed. Don't leave a ship with low hitpoints to shoot at you through the entire battle. Watch you Dont turn the hole fleet to deal with 1 ship behind you.

focus fire ur ship groups to their strengths.

scouts, flak, HC and fighters attack LRF

bombers and LRF attack capitals or LF

LF attack flak or carriers

etc...etc...