If you trypsinize, and add the ethanol cold, dropwise, while vortexing,
you should be able to eliminate aggregates, as you do by sonicating the
yeast.
Some people, instead of using EtOH permeabilization, use detergent
(NP40) to stip the nuclei out of their mammalian cells. This "Vindelov"
protocol can be found in J. Paul Robinson's methods book.
You can store the EtOH treated cells in the cold if you wish, but many
people have found that they reamian stable for a week or longer (in
EtOH) at room temperature, in the dark.
Mark