I'd usually just stick with BPA but if you're presented with an opportunity to stock up on high end forwards, odds are you'll need to get them early. Whereas quality defensemen have historically been more attainable in the 2nd round. It's something to consider I would say.
High end forward meaning centre ? I will assume that.
This is actually quite true. If you take a look at some teams with enviable Ds like Boston, Carolina, LA, Anaheim etc you will see that the top 4 includes players from a range of rounds.
Top 6 centres however are typically uncovered within the 1st 2 rounds , like 80% of them and 1Cs typically in the top 10 or so.
Bottom line is NHL scouts are very good at identifying centres so they go quickly, top 4 D on the other hand, not anywhere as efficient.