I always thought it best to divide among the three skill trees, but was encouraged to concentrate on just one, as the most powerful skills at the top were crucial to victory in the endgame. Here's my personal construction for a Jedi Sentinel in the works.

Jedi crusader is fine for levelling and but during endgame Master Focus and Valour are worth much more. See this thread from our Sentinel mentor who specialises in Combat https://swtorponies.forumotion.com/t814-mentor-pve-sentinel

There is really only 1 spec for endgame combat and it's 5/31/5.

The only time it's ever good to divide among the skill trees is for PvP:

http://swtor.askmrrobot.com/skills/sentinel#c-2dec16ekk-120ek
http://swtor.askmrrobot.com/skills/sentinel#1728clc-1ec16ekk-0
http://swtor.askmrrobot.com/skills/sentinel#1628clc-0-c2f1e82k

Those are the 3 I'll sometimes use.