The negative usually pops upwards as the enlarger head warms it up, so one glass on the top is usually enough. However, if the film base is soft (and thin), you will get slight bending of film downward due to the gravity. This is the reason I am switching to T-Max, which is stiffer than Fuji Acros... However, if you place the negative into carrier turning the carrier upside down (so the AN glass is at the bottom), putting the film into the carrier and closing the carrier in this position, the film will be kept by the carrier in considerably flatter position than if you just put it in in usual way. Even soft film base will be fine this way (of course this all depends on the way the carrier presses on the film when you close it, so this may vary from enlarger to enlarger).
Joseph