The gridlines can onky be used when you have a horizon or some exact verical lines in your picture. You have to place the horizon or the vertical lines over one of the grid lines. If you want it somewhere ele in the pictude th gridlines will not be very helpfull.
In many situations you have no good view of the horizon, like in a city, inside buildings, in a wood, in the mountains, when taking macros etc.
so, yes the gridlines can be helpfull, but not in every situation, the level will alwais help.
For the accuacy of the level, I can tell nothing, as my Nex 5 does not have this feature. I can not think this level is good enough to rely on itbas a building tool, but I think you can easely rely on them for photography I hope...