Yeah you should if you can... the stockers run 90amp I think. It won't hurt, but you'll run through batteries and alternators a little more quickly. I've never changed mine but ican definately tell when that fan comes on. Combined with everything else that's a lot of current for a little alternator.
Actually the stockers are 65Amps. Later model cars ('91-'93?) ran a 75Amp unit. But still this wasn't even enough to power all of the electronics in a stock car. JRichker has a great write up on the total current draw from the stock electronics. IIRC it adds up to about 75Amps so depending on what year your car is, you are already overextending your alternator.
www.pa-performance has a small case 160Amp alternator that you can just drop in without having to do any grinding of the stock bracketry. The only thing you need to do is run some wiring to avoid starting a underhood fire.
Can you get away with not upgrading the alternator? Yes, but will your headlights dim? Yes and your poor alternator will probably die a painfull death. If you have the funds, upgrade. If not, start saving becuase you'll need it.....
Use these sites for information on the right way to do the wiring. Some people will tell you that you can skip the wiring upgrade, but it will catch up with you sooner or later. A fire in the wiring harness is ugly and expensive.
Under no circumstances connect the two 10 gauge black/white wires to the 3G alternator. If the fuse blows in the 4 gauge wire, the two 10 gauge wires will be overloaded to the point of catching fire and burning up the wiring harness.
The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.
Electric fan = 3G alternator if you want long life & reliability from your car.
The electric fan saves some HP. The stock fan's parasitic drag runs from 7-12 HP depending on who you talk to. The electric fan uses about 1/2 HP of power from the electrical system.
Figure this:
Ignition system & computer = 12 amps
Fuel pump = 12 amps
Exterior lights = 15 amps
Fan (heater or A/C) = 15 amps (can run between 5-25 amps depending on setting)
Radio & instruments = 10 amps
Wipers = 10 amps
That's grand total of 74 amps from a 65 amp alternator. Talk about overdrawn at the bank!
Any clean shiny spot you can find on the engine. I bolted mine to the A/C-PS bracket. A better place would be the battery to starter ground near the front timing cover.