Slug Gun - since this is for a .30-06, with a longer throat (longer than the abrupt .30-30 throats) I'd vote for whatever size fills the throat. You could start with an unsized bullet seated out in a dummy round, close the rifle on it to get some throat impression, and measure the back end of that impression. The goals are to seal the throat and provide best possible bullet guidance into the barrel.
Don't be surprised if this means no sizing at all.
To illustrate, I had a 9x56MS with .353" bore and a long throat that began at .359". With #350457 (.354") accuracy was OK with light loads but fell apart at hunting velocities. With #3589HP, the same story sized .354", but good accuracy (1.25" at 50 yds for 5 shots open sights) with bullets at the as cast diameter of .360" and loaded to 1900 fps (17.7" barrel).
The .30-30 is in my view a different story, since it has an abrupt throat (if you can call it a throat) offering minimum guidance. The solution for the .30-30 is to use the case neck for guidance. To do this, I load .3125" (Lee .311" die) in .30-30 for both full throttle and light loads. This is running close to minimal chamber clearance, and the odd case may be tight. To nip any problems in the bud I do a light ONT, scraping off the high spots, picking up any thick necks, and leaving .010" max neck thickness.
Short answer: Fill the throat for anything with a proper throat, or fill the neck for an abrupt throat.