I only use use spirit level on the levelling base when stitching panos - unfortunately through the years I haven't found a good leveller (even the more expensive ones don't seem to be spot on, despite what many will have you believe, probably because there is a wide margin of error [the Sirui ballhead levels are quite ok, but getting them right is a pain in the arse because of the high sensitivity, if you know what I mean]), so I use the live view and "zoom in" to check the parts which aren't affected by this by eye. With the grid overlay option present in most cameras, this is usually quite easy. In any case, when it comes to cityscapes, the verticals will tell the story - seascapes are a bit more "forgiving" in this aspect (also much easier to liquify).