The top 4 positions of need for Dallas, in no particular order are:
LB
CB
S
DT
I don't like to pigeon hole myself by saying this rd take this and this rd take this, but if I were to rank which round I think the best talent/value ratio is for each position, I'd go something like this:
RD1. CB/LB - I don't really see any rd1 S or DT this year so grabbing a top CB or LB would be my move.
RD2. CB/LB/S/DT - Rd 2 a good player could be taken at every position. But the top CB/LB/S should still be the target.
RD3. S/DT/LB - I think Rd 3 will be a very good spot for a quality S or DT this year.
*RD3. S/DT/LB/CB - I think we're getting a 3rd comp pick this year, fingers crossed!
RD4. DT/S/LB - Hopefully you got your CB early so you take whichever spot you missed out on up to this point.