Proper lead shot placement for float fishing is probably as more if not more important than bait selection when float fishing for any species. Assuming you are "where the fish are" then "presentation" is everything. You can have the greatest jig, bead or bag of all time on the end of your line, but if your presentation is not in the right place doing the right thing, then it does not matter if you are fishing with a bare hook.
Shot patterns and placement are more than just "getting a bait down" into the strike zone. If that were the case, we would all be float fishing with 1 oz egg sinkers on our mainline that drop our baits like a rock into the strike zone. The proper "presentation" for float fishing is all about proper shot selection and shot placement....depending on water clarity, depth and flow.
Using the appropriate shot size and placement allows for a more "tapered" presentation, similar to the effect of "checking" or "trotting" your float, where as your presentation swings up and back slightly away from directly underneath the float. This tapered presentation effect is one of if not the most effective float fishing presentation method, as compared to a completely vertical float presentation with your bait directly or even in some cases dragging upstream from your float.
The reason the tapered presentation method is the most effective is due to the difference in current speeds at different depths. Typically, current speeds are higher closer to the surface and slower near the bottom. Naturally, baitfish, bugs, eggs and other potential food items for salmon and steelhead will settle at this lower layer between the faster moving mid water and top current, and the slower bottom currents. Steelhead and Salmon will lay in this zone, picking up easy offerings as they settle at this layer as they float by.
Your float fishing presentation with a tapered method will ensure that 1) the first thing the salmon or steelhead sees is your bait...not your shot, swivel or float and 2) your bait presentation is not dragging upstream of your float, and not in the ideal strike zone between the slower low layer currents and the faster mid water and top water currents.
When your float is moving further downstream then your bait presentation (and you believe it is straight vertical), you are essentially "dragging" your presentation through the water rather than letting it more naturally "float" through the hole in the strike zone. The natural presentation is produced by properly spacing and sizing of your shot, and "checking" or "trotting" your float....not to mention finding the seam between currents on a vertical plane.
Below are examples of some of the most often used shot placements and their respective water flows/depths in which they would be used.
Fast Current and Deep Holes
Standard Shot Pattern for medium speed and depth