Quilting that quilt
 

I am about to embark on the biggest quilt I have ever tackled. I plan to do individual motives in the 12 inch
blocks. My question is should I ditch quilt as well. Thanks in advance, Rae.

It's hard to say without seeing a picture, but it's a matter of personal preference. Many quilts look good with stitch in the ditch to help define the blocks. The benefit to stitching in the ditch is that once you have done it, you can start your block quilting anywhere you want.

I will frequently SITD just for stability, then do the individual blocks with particular quilting motifs.

if your motif is not large enough, the space between them may be too wide for the batting's requirements. in that case, i would definitely SITD

Look at the packaging from the batting. It will tell you the maximum distance your quilting can be. If your motifs are big enough so the unquilted area is less than what the package says, you are good to go.
Having said that, I agree with NJ Quilter. If you SITD first it really does stablize the quilt making it easier to go back and fill in the blocks with the motif.

Thank you ladies, I will ditch first simply to make it easier on myself.

I'm working on something like this myself right now. I made a large 94 by 81 Queen sized quilt and am ditching each of the blocks...it's on the frame now. I'm so happy I did too because it just looks so much neater and also does really help to stabilize the blocks. It's extra effort...but worth it in my opinion! :)

I stitch in the ditch to stabilize my quilt also - then quilt the blocks with other designs. That way I can take out the rest of the pins and don't have to worry about the quilt moving.

Quilt the main seams in the ditch to stabilize the quilt and then do your block design. If quilting in the ditch interferes with your overall design, SITD with water soluable thread. When you wash the quilt those seams you don't want disappear.