Spamming corporations into other civ's cities can be very dangerous, unless it's a special case like Aluminum Co or Standard Ethanol in a civ that already has access to these resources. +5 income per city is usually not worth giving your opponents the substantial advantages that corporations offer.
Putting as many corporate HQ's as you can in the city with Wall Street really makes a difference, as does running Free Market. Mining Inc is certainly worth putting in your highest-production cities, and it comes early enough to make a difference. Sid's Sushi is also a good choice, providing culture and food bonuses; this is one you may want to spread to as many cities as you can afford. Civ Jewelers is very nice as it's the only coporation you might not lose money on, but it comes very late and competes with Mining Inc.
In my current game, I have 8 cities; I have Sid's Sushi in all the cities, Mining Inc in the 4 best production cities, and Civ Jewelers in the rest. I also founded Aluminum Co, just to keep it out of the hands of my chief rival Persia (who doesn't have aluminum). Just for fun I spread Sid's Sushi into one nearby Persian city, and Darius promptly spread it to absolutely every one of his cities. So my Wall Street/Corp HQ city has an income of 580, and I'm easily able to run at 90-100% research.