Doesn't necessarily have to be 1B, though 1B and RF are the two positions where you can sometimes get away with a D+/D/D- fielder and not get killed (Babe Ruth & Gavvy Cravath, I'm talking to you). Remember that the key to using scrubs or AAA players as regular position players is stamina. If they can't give you close to 600 PA's for the season, you'll wind up with a huge hole in your lineup.
I usually don't like to gamble on AAA players blindly right from the start of the season, but that's just me. If I have a AAA player with "A" stamina and he seems to be able to hit above some arbitrary Mendoza line like a .200 BA, I'll drop the position player at that position early in the season for waiver wire upgrades. I usually do better when I drop him for a couple decent upgrades rather than one flashy stud.
In other words, I'll drop a $5 MM position player for 2 upgrades of $2-$2.5 MM apiece at other positions. Remember that you're paying a 10% penalty tax for using the waiver wire, so use it sparingly and be patient. Don't make your add/drops until a player you REALLY want appears on the waiver wire.
Always make sure you have scrubs with the highest PA (e.g., Hughie Jennings) available so you have the roster flexibility to use the waiver wire (SIM won't let you make add/drops unless you're keeping your roster above a minimum 4800 PA's).
Try to avoid judging players by raw stats -- because of normalization, different ballparks, lineup configurations, etc., they're usually misleading. Try to rely more on the SIM performance histories of each player, but also use your own common sense.
Those are all the trade secrets you get free of charge.