i have a question

when should you defend your blinds and should you not???
i play mostly mtts and sometimes sngs and cash games

im guessing in cash games you fold them a lot

when i play mtts i usually defend to anything smaller than 3bb and i would say i defend with 50% of hands a 3BB with one middle/ late raiser and from small blind i hardly ever fold if there wasnt a raise
then i noticed when watching some mtt videos that most people fold their blinds to any raise
so lets say you have something marginal like k7s or 910s...would you fold your bb to a late midle position raise?? and why???if you dont do you call or 3bet?

tnx in advance