concerning the slope for the flow frame extraction: there is no problem with putting some wedges at the front of the hive and leaving them there permanently. It is important to keep the hive perfectly level from side to side so that the frames hang straight down (especially when the bees are drawing out foundationless combs- they use gravity to guide them and if the hive is tilted to one side all the combs will be too)- but it doesn’t matter if the frames are tilted lengthwise. Also you can put in a bit more of an angle than the angle built into flow hive bases- that helps the honey to drain out faster. This is especially good if your honey is very thick like mine is. Last time I harvested it took more than 4 hours for all the honey to drain out. I have put permanent wedges under my flow hives to increase the angle by a few more degrees.
Concerning the bees and the flow frames: I believe that rubbing wax helps give them the idea- but that the most important thing is that the bees have enough spare nectar to need to use the super. I have had bees fill out supers without rubbing any wax on them- and I have also rubbed wax on the frames and the bees have still ignored them until they are good and ready. The frames can sit unused for months- and then suddenly fill up when the bees have built up enough and get onto a good nectar flow.
EDIT: see below- what I wrote is more relevant to hives with screened bottoms.