Generally there are three schools of thought with regards to the usage of mainlines and leaders when float fishing. One school, and probably the primary school (certainly in Ontario and with most Centerpin fishermen) is to tie the mainline directly to your leader/presentation. That would be, float and float stop on the actual mainline..hi vis or not. From this camp, philosophies branch with regards to the sight factor of a mainline. I.e. can fish see the mainline?
Camp A says 90% of what everyone thinks they are doing with float fishing is completely wrong in trying to keep the float completely vertical and moving at the rate of the current. These guys believe that the rate of float should be slower than the rate of current, to match the slower bottom currents where 90% of the fish hang out. That would mean your actual float is at nearly a 45 degree angle pointing upstream, which would also mean that the mainline is completely outside of the vision cone of steelhead regardless of depth or water clarity. This is called trotting.
Camp B of this branch says you have to stay vertical with the general rate of current, which would mean that steelhead "could" in fact look up when approaching the presentation and see a hi vis mainline. This camp (mostly in Ontario) believe this to be true, and therefore have an almost religious belief in tiny diameter mainlines so as to remain as "stealth" as possible both with the mainline floating on the surface and traveling down through the float and tied direct to the leader.
The second school of thought is that steelhead CAN see the mainline (again on vertical drift..as this is the prevailing presentation method), and that it can be very critical to splice in a short section of "clear" line, typically mono, for the float and float stop. This would push the higher vis mainline further upstream and outside of the vision cone in shallow clear waters.
In speaking recently with Pro Staff Kyle Buck of Great Lakes Guide Service in Michigan, he recounted a recent outing where he was getting his rear end handed to him by a fellow angler on his boat who had "spliced" a section of clear mono and after making the change himself, caught up in the catch score.
The third school of thought is to play the clarity and vis game on a daily basis, and based upon depth and clarity of the hole you are fishing, make the adjustments with your presentation and add a splice piece accordingly to keep the hi vis mainline out of the vision cone of fish.
Ultimately it depends on conditions and personal preference. If it is easier to see your high vis mainline to help in mending your line on a drift...the hi vis is the way to go. If you are a trotter, or pressured fish/clear water angler....stick with the low vis main line to give yourself the confidence to hook up.