Is there any advantage to preparing the tire bead with sealant during installation on the rim
The soap and the sealant do not have the same purpose.  

The purpose of the soapy solution is to lubricate so the tire better pops (seats) onto the bead.
For some tires it is hard to get them on the bead (especially with a hand pump).     

From there the sealant in the tires is used to seal.  

Stans has some good videos
StansHelpCenter

tireinflatingtips


  Use soap and water to help the tire bead seat.   
  
  Do not add sealant until you’ve successfully been able to seat the
  tire.


I get the thought of have some sealant on bead.
But I will go with getting the best seat (soapy solution).
And that is what stans recommends.  

Other tricks are:
Shake the tires to disperse the sealant.
Lay on each side for 12 hours.
Run em up to 40 PSI and be sure they hold for 24 hours.
Some tires suck up a lot of sealant - you may need to add sealant.