I'll put it this way if both people spoke their mind all of the time during their "courtship" most likely there would have been less marriages! LOL! It seems the longer a relationship goes on the less "editing" takes place in conversations between a couple.
Actually it's not what you say but how you say it that counts the most. Right off the bat when someone says, "Speak your mind" they assume you're not going to be tactful or you're going to resort to being rude, cursing, and rasising your voice.
However speaking your mind is simply being honest about how you feel about something. One can do that in a matter of fact tone without exploding into a tirade, attacking or belittling their spouse's idea. Honesty does not have to be mean.
Without honesty there can be no trust.
Not many people would choose to marry a liar.
If you disagree be prepared to offer an alternative solution. Offer constructive criticism.
I'd personally like to know exactly where I stand with my wife.
But by the same token I also subscribe to the "Don't ask, Don't tell" policy if you don't think you can handle the truth!