opposition instructions......?

if my CB's are set to tight man marking as my general tactic, do i still need to apply the tactic to the opposition strikers before the game?

Depending on the opposition formation, your CBs will tight man mark the strikers facing them. If the formation changes, they'll switch the target of their marking to the most appropriate player.

The Opposition Instruction tight marking instruction means that no matter where on the pitch that player is, the most appropriate player on your team will attempt to tight mark him, regardless of their own instructions. If you place this instruction on the strikers, it normally applies to your CBs, but if the AI switches the striker out to the wing, for example, it then becomes your fullbacks responsibility to attempt to tight mark that player.

So to answer your question with a question, do you want the strikers tight marked regardless of where they are on the pitch, or do you want your CBs to tight mark whoever is in front of them.