When you say foreclosure- i'm assuming the bank has regained ownership of the property and the owners have no way to redeem it at this point. I am not referring to properties where a LIS or NOS has been issued but the owner still maintains control of the property up to the time of sale.
It just depends on the source of the house. If it's a bank REO- you will only have access to it once it's listed w a agent - at which point every investor and their realtor can see it on the MLS. I'm not a fan of wholesaling anything listed on MLS. I've tried to no avail to relieve the bank of the property prior to listing. I don't know many wholesalers who try to flip these to investors since the investor already has access to the property.
If you are buying a HUD listed house and are the winning bid, you can put down your EM and market it while you are waiting to close. You can not assign the contract so will have to close yourself w HUD and a second time with the end buyer. I know several wholesalers who do this.
If you are buying at an auction or county sherif sale- you can buy with your own cash and resell to anyone you want in whatever condition you want. I know several wholesalers who do but you again need your own money to close.
The reason they stress pre-foreclosure over foreclosure from a wholesale perspective is because as you can tell from my comments above, it take actual cash you possess to close on a foreclosure. With a pre-foreclosure you are still dealing with current owner who isn't care if you assign the PA.