It depends on so many factors. Mostly you want to get rid of evidence that you put it on the lathe after your done. People used to either fill the screw holes or cover them with green felt. Most turners don't do that any more because it looks very amateurish and we have better ways of doing it today. Here is a link to our club website tips pages. I wrote an article on Methods of Reverse turning bowls.
Now back to the problem at hand. Screwing directly to a faceplate is the most secure. I drill out extra holes in my faceplates because 4 is not enough for larger pieces or long pieces of wood if your doing hollow vessels. I also counter sink the holes on the side that faces the wood. When you screw into the wood it wants to pull fibers down and can leave the wood not sitting solidly on the faceplate. counter boring the wood side of the faceplate leaves a place for these fibers to go and makes a more solid connection.
Of course using a faceplate with screws means that you must have enough wood on the bowl blank to be able to cut that wood away when you finish the bottom. This really only takes a little more wood than turning a tenon for a chuck. I try to leave the foot of my bowls either larger or smaller than where the screw holes would be so that they can be turned away when I reverse turn the bowls.
Using a wasteblock eliminates this problem but you must use a good solid piece of wood for the wasteblock. I use scrap maple or Poplar most often. If the bowl blank is dry you can use any wood glue. If it's green then you need to use a polyurethane glue. If your skills are good and you don't get catches you can use CA glue but the joint is brittle so I don't recommend it for new turners. You will need to wait 12 hours or so for the glue to set. The surface of your bowl blank and wastelblock must also be perfectly flat so you have a good joint. If you don't have the tools to make the bowl blank flat you might be able to use a thick epoxy to glue them together since epoxy is a gap filling glue.
I used to turn bowls this way. Screw the faceplate into the top side of the bowl blank. Then turn the outside of the bowl leaving a tenon on the bottom of the bowl that is slightly smaller than your wasteblock. You can design this tenon to be the foot of the bowl if you want. Finish the outside including the foot. Flatten the bottom surface of the tenon. Now put your faceplate with wasteblock on the lathe. Turn a depression (we call it rebate in turning terms) that exactly fits the tenon on your bowl. It only needs to be 1/16" deep. This exactly aligns your bowl when gluing the bowl to the wasteblock. Make sure the bottom of the rebate is flat so you get a good glue joint. Now glue it and let is sit overnight and your ready to finish the bowl.
When you have it done use a parting tool to cut down most of the way through the tenon leaving the foot. I stop the lathe and handsaw off the last inch. If you want the bottom flat just carve or sand this part off. If you want the bottom recessed then use one of the reverse turning methods mentioned above.