you dont always have to do that, there are a number of different ways that you can transition into a new song. the bass flip is just one of them, simple and quick.
I usually turn down the bass when both songs have high bass, it sounds terrible at partys. Especially when I use 2 bass'. I turn it down on the one I am already playing and keep it on the next song I'm transitioning to.
It depends on your transition. If the 2 songs have a heavy bass line turning the outgoing one down will help or else you can get a muddled sound. But there are many ways to transition and thats part of practicing and knowing what transitions you are capable of. The more transitions you can do the more versatile you will be.
Its depends on the songs you are mixing... different type of transitions require different techniques.... you can lower the bass a little, to get that full sound while the bass on the incomming track is normal...
It really depends on the songs and how the bass lines blend...if you have 2 songs you're mixing you should ideally turn one down so that they don't clash/sound muffled...but as some of the guys said it's what effect in the mix you're looking for...
All depends on the song and the beat. If you could match it then dont worry about it but if you cant slightly turn it down without killing the riddim
Depends on your genre. With EDM Genres such as Electro, House & Trance you should def be EQing bass, as well as highs & mids during your mix. As for genres such as Reggae/Hip Hop there is no need, other than if your music collection is of various encoded bitrates/formats and you need to use your EQs to turn frequencies up or down to balance levels of tracks on your playlist.