plentivirus - stable cell line (Apr/11/2008 )

HI I am using plentivirus to generat stable cell line. the shuttle vector contain blasticidin to selecte the cells containing the transgene.
I know some people use shuttle vector containing GFP but no blasticidin and enrich the GFP cell with facs machine. Since there is no blasticidin you can't keep the selective pressure on the stable cell through blasticidin. So does the blasticidin is use as selective agent or as selective pressure.Meaning does the transgene can stay with out blasticidin selection over time In this case using only GFP will be ok even over time.

thanks