I think it's okay for someone to ask if you can check out their videos, but I don't like sub4sub because i'd rather have lesser numbers with more interactions than the opposite.
Asking people to check out their videos is just as bad. The majority of the people who do that stuff usually just spam it to everyone, and make up some b******t like "I need feedback and stuff, yo." They don't actually interact at all with you, this is speaking from experience with these people.