How important is mixing in key?

As i developed mixing skills i use my ear to decide if the key of the next track is compatible.Id say its just in your mind if you can recognise the key.For example when listening recently to some sets i did in late 90’s i cringed upon hearing the change in keys on the track transitions…..but at the time i didnt notice as i was as less experienced.I think if you can sing in tune,in the correct key,you should be able to transfer this know – how onto your mixing.