Should i avoid some teachables ?

It's a matter of personal preference. If you're very concerned about unlocking specific perks and don't want your bloodwebs diluted with perks you'll never use, it's find to hold off on getting them.

Personally, I would advise you to just get them so that at least you have them in case you want to try an interesting build, or maybe if they get buffed in the future. The truth is, there is no such thing as a worthless perk, so unless you only ever use the same four perks and would never consider anything else, I'd say most of the time even the weaker perks are worth owning.

However, having said that, it does depend on a couple of things. For example, if you have only leveled that character up for their teachables and don't plan on leveling them further, there's no harm in waiting to get the weaker teachable perks until you have the perks you actually want on your other characters. Then you can buy the teachables and unlock them on your other characters afterwards without worrying about diluting your bloodwebs.

If you are planning to continue leveling the character whose teachables you are considering ignoring, I would advise you to just get the teachables, because otherwise they will continue to show up every level in slots that could otherwise be filled by different perks that you want to unlock.