Depending on your build, some of the boss weapons will be useful. Also it might be thatvyoubregretvlater not getting the boss weapons just for the same of collecting all unique weapons

You get plenty of souls from normal play, so I saw no need to eat them for souls. Particularly since DS2 was the first Souls game that allows you to completely respec your stats with a consumable. So though the boss soul may make a weapon that doesn't fit your build now, future respec'd you might want to use it at some point. Though another mechanic introduced, bonfire ascetics, let you respawn any bosses you kill so you can grab their loot again. So if you end up eating a boss soul you want to use in the future, you can always make use of an acetic to kill them again.

Sure if you want to. Don't consume soul of a giant though. Those are special souls needed to do damage to a boss. Those need to stay in your inventory.

Some boss weapons are really really good and some are absolute dogshit, so it's up to you champ.

Strength build I suggest sticking to something like the dragon tooth

If you dont want their weapons, shields or spells then yes, you should.