Buried in the ol' Q&A sessions I remember the question coming up a few times and what I wrote down then I still use today. It's pretty easy, intuitive, and logical.
QB Position;
Throwing- Only QB's (or those occasional strange people who give their punter or HB throwing.

)
Running- HB's are the best (least penalized). RFB's likely a close second but I've never tried it.
HB Position;
FB's are the top choice. Pick the one whose job you want at that spot, in that play. ScatFB for a passing play for example. WR's can play here as well if you don't have a Scatback. Penalty is slightly more than FB.
Center Position;
Guards. Period. I've never tried Tackles but I suspect the penalty is only slightly more than a Guard's is.
Guard Position;
Tackle or Center. Penalty for either is same.
Tackle Position;
Guard or TE (are you sensing a theme yet?). Penalty is the same.
TE Position;
Tackle or FB. Penalty is the same (I'm 99% sure).

TBH. I've used FB's (especially ScatFB's) there when I've had no receiving TE's and they worked fine. Tried WR's and it wasn't so good. For blocking, especially Goalline formation, sometimes a great tackle is perfect if you only have 2 BTE's and you need 3.

WR Position;
Depending on WHY you're playing a non WR depends on which oop dot to use. For Blocking, a BTE works decently. Haven't see anything else work well out there for that job. For Receiving, either a Scat HB or a RTE. Both work about the same and work pretty good.
As Slughead points out, the farther away from your natural position, the worse the penalty. Below are the defensive ones (shorthand)
DE for DT's
DT's and LB's for DE's
DE's and SS for LB's
LB's and FS for CB's.
CB for FS
LB for SS