I always upgrade everything as soon as possible for a couple reasons. The stackable moodlets are great to enhance other moods for one thing. Plus it's great having your Sims hygiene go up when they use the toilet. Sometimes they don't even need a shower afterwards and it also cuts the time needed to use those things almost in half or even less. Lastly, because I absolutely adore having the Retro station on stereos to hear TS2 songs.
Oh and just a tip for those who don't mind doing something a bit cheaty to get free parts. You don't have to spend thousands. All you have to do is scavenge for parts then click to throw away. BUT, immediately cancel the action and they'll drop the bag allowing your Sim to scavenge again. You can do this over and over for as long as you want to get more parts. Just make sure their trash can is far away. ;)
The only drawback is the time consumption. I think it's worth it though if your Sim doesn't have a lot of simoleons to spend on parts or enough satisfaction points to spend on rewards to eliminate those needs.
EDIT: One more thing, I never get the thermostat for my Sims, they all have a fireplace to keep warm and it's always on except in late Spring/Summer. I have not had one fire yet and that's one thing I've never upgraded. I didn't even know it was upgradable until I read about it in another thread.