It's all a matter of what is comfortable for you.
I personally find it easier to move the fabric left to right when stippling or meandering. So the feed dogs engaged would not allow me to move the fabric freely sideways, as they feed the fabric up & down.
When doing feathers or other designs where you are following a pattern, the feed dogs can also interfer.
This is the way I set up my machine: feed dogs down, adjust the pressure on the foot by lowering it by 1, new needle. Less pressure on the foot allows you to move the fabric freely. I use an darning foot that has spring action to it. I do a test to see if the tension is ok, adjust it if I need to.
If I want to FMQ in the ditch, which I do sometimes, I do leave the feed dogs up to help feed the fabric. Remember that the feed dogs are moving the bottom fabric, so watch that you do not get puckers in the backing if you leave the feed dogs engaged.
Hope this helps.