Not including wheat pennies or earlier, I only keep the early ones, low mintage memorials, the proofs, or any errors
I define the early and low mintage ones as follows:
1959, 1959-D, 1960, 1960-D, 1961, 1962, 1963, 1968-S, 1969, 1969-S, 1970-S, 1971-S, 1972-S, 1973-S, and 1974-S
Then again, I look through too many pennies to keep all of the pre-1982 pennies. If I did save all those, I would have saved approximately 23500 pennies already in 2007. That is based upon an estimated 600 pre-1982 pennies per $25 box with just over 39 boxes searched. At that rate I would have $1000 in pre-1982 pennies at the end of the year. Instead I have kept just over 1200 (w/~500 S-Mints) of the early/low mintage memorial pennies this year to go along with my 372 wheat pennies.
So basically by not saving all pre-1982, I have an additional ~$220 to go buy rolls half dollars, nickels, or more pennies and find other coins of interest.
I prefer to cycle my search money searching for something that is more interesting/valuable today instead of saving it for a potential return in the future which may not materialize. If the marketplace changes, I'll have to reassess.