Depends on the type of insulating fluid, as we know, almost all samples are performed from the bottom because appart from "askarel" all other insulating fluids have a specific gravity lower than 1 (wlower than wter's SG) which will let the waer sit at the bottom but in older transformers with "Askarel" as Insulating fluid (with SG, of 1.3 - 1.5) the water is lighter which is why sample port is up top.