rcavictim got it in one. For best effectiveness with a magnet you want:
1) reliable flow
2) at a low rate
3) in a thin layer
4) over the magnet
Putting them in the bottom of the barrel will make sure that anything that settles out is retained (certainly a good idea), but it will provide no guarantee that anything entrained won't stay in the oil. There are ferrous fines that are small enough that they will stay suspended in oil for months or years, and they are nothing but bad news in fuel.
Probably the perfect magnetic filter would be a thin rectangular cross-section passage with magnets on one of the long sides. As long as the flow rate and viscosity were low enough, you'd get essentially 100% of the ferrous fines onto the magnets.
On transmissions and splash-lubed crankcases, I put the magnet(s) inside in the splash pattern. Usually there's an inspection cover somewhere that makes a convenient magnet mount. I'll be putting several magnets in the bottom of the sump of my 6/1, and probably at least two on the inspection cover. On hydraulic systems, I stick them in the reservoir near the return line entrance. I stick them on the outside of spin-on oil filters, and in the return oil flow pattern on pressure-lubed crankcases.
There's no downside to the things as long as they're properly attatched. The worst thing that could happen would be failure to clean the swarf off in a sufficiently timely matter resulting in a chunk breaking off and washing around in the lube. That's one reason I generally put these where they can be inspected easily, or on a remove&replace component like an oil filter.