I usually devide my fort up into 4 districts: a slum, a middle class district, a high-class district, and Noble Quarters.
Slum rooms are little 1x2 affairs. Just a bed and a door, no smoothing, no engraving, and no furniture. It's tight, compressed, and usually empties out later on, only holding migrants and skilless peasants. It's the first set of rooms to get doors, though, so for a short time it's a luxury just to have a bedroom with a door on!
Middle-class housing is 3x3. Initally it's just a bed with no smoothing, but eventually I put in basic furniture (cabinet, coffer, maybe a chair and table) and smooth the floor (rarely the walls. The way smooth walls interact with rough walls bugs me, so unless I smooth the whole fort...). Sometimes I'll engrave things on the walls, since I have the visible engravings option on and it meshes nicer with the rough walls.
Upper-class housing is for the mayor, manager, book-keeper, broker, and their ilk, as well as legendaries who have useful skills. 5x5 rooms, with smooth floors, a cabinet, table, coffer, chair, armor rack and weapon rack, and whatever other luxuries I see fit to give them. They rarely get statues; however, I usually double up the table and surround it with chairs, to make it feel like they're ready to entertain guests. They get engravings too, and by this point my engravers are quite skilled.
Nobles get unique houses, usually sprawling palaces with branching rooms and hallways. They have smoothed floors and multiple engravings, plus statues and other decorations. If it's a big noble, like a count or king, they might have pools of water or 3-D rooms. One time I even gave my dungeon keeper a special seat at the Arena; windows along the bottom! While the commoners would sit and watch from the stands, and the Baron and Baroness were watching from the noble booth, the Dungeon Master got a chance to get up close and personal with clearglass windows looking into the arena... which was never used but whatever. It doubled as his dining room, too!
Someone mentioned tombs as well. My commoners are buried in communal tombs, a 10x3 block, but with notches de-designated from the long side to bury about six dwarves each. Again, the floors are smooth, and the wall behind each coffin is usually engraved. I also bury important or "rich" dwarves in 3x3 tombs, personal, this time. Nobles get large tombs, usually with fancy designs and traps in front. Most have an entryway of sorts, often with engraved pillars that either detail the life of the dwarf that is at rest there or the life of some random bloke who got a lucky shot on a badger that one time. I mean it was awesome. That badger didn't know what hit it. But we do. It was Monom. Monom hit it. It was awesome.