>>271932182
think you have enough time to make it to the site, fight the planters, and defuse? more than likely you should go for it. is your team already way behind and/or there isn't enough time to complete your tasks while still having 9 seconds or less to defuse the bomb? go around the map quickly and pick up good weapons for your teammates. they'll appreciate not having to eco and only worry about armor more than you rushing a site that's already holed up.
you can also try camping an exit from the bomb site after picking up weapons to try and catch any escaping T's out!
a lot of this is stuff you'll be able to pick up after playing long enough (do you have time/resources/life lead to take the site, should you try, how your team's economy is, etc.) instinct and feeling play an important part in quick decision making!