Good question. First, I'll give you the criterion I use to determine what to do... Personnel, Position, Tempo.
From the personnel aspect, I try to put the best 11 players on the field. "Best" depends on my goal AND my plan to achieve it. Sometimes its better to have a FB block than to use 4WR... Basically, use someone that can DO THE JOB.
On the position side, formations place players in key positions pre-snap so they can do their job post snap. Case in point, if the defense blitzes the middle, there's no sense placing additional blockers away from the middle. At the risk of sounding condescending, blockers block better when they are CLOSE to the guy they are blocking.
On the timing side, we should realize the connection between personnel and position. Blitzing with fast players close to the LOS mean quicker pressure than if those fast players were 10 yards deep. Likewise, slow blockers must be closer to the place where they will block to get the best effect.
That said, I use I-Form because the FB (personnel) picks up middle pressure from a spot (position) that allows the QB time to safely take a 5-step drop (tempo).
When down and distance require more receiving threats, I still like to use I-Form (3WR) (personnel & position). Block with both backs and let the Receivers get open, using my QB's feet to buy more time (tempo).
As long at you think about your 'moves' in terms of all three elements, you'll be OK.
Here's an article called "
16 Ways to Blast Blitzes" that sums up many ways to beat a heavy pressure scheme. You'll need to sign up to see it, but membership is FREE.
Later