Ive been mewing for about a year and for the past month or so i mewed with teeth barely apart because I have a deep bite so i theorized it would be more natural and that it would allow more tongue to be on  the roof of the mouth. Although it did seem to let me put a little more tongue on the palate ive conclued that at least for me its not very beneficial for mewing. I went back to having my teeth slightly together and i noticed my posture improved dramatically and i felt a lot more force on my palate. I think having the teeth together is a vital part to mewing it seems to connect everything together. I have seen some success stories on here by people that dont put their teeth together so dont take this as concrete advice it was just my experiment