Very few saxophone players know how to use their altissimo register tastefully. Sanborn ruined legions of alto players who tried to copy him. Before Sanborn was our man Sigard Rascher, who still tortures gentle listeners the world over with his groundbreaking Top-Tones book. I prefer the lower register of the alto saxophone to the upper extremes of the instrument. When I listen to recordings of myself playing upstairs I ofter regret that I choose to make that trip to the attic, even if I was perfectly in tune. I can stomach the tenor sax's altissimo a bit more, since it is somewhat lower and darker that the alto.
The thing that I don't usually like is when players just pop up to the altissimo range from lower parts of the horn, suddenly screaming out a double high D that makes all the dogs for a two mile radius go nuts. The least annoying way to use the castrado range of the saxophone is when you make it a natural extension of the normal range. A few guys my age who do this well are Mark Turner, Matt Otto, and Doug Yates. They usually approach altissimo in a way that doesn't make you cringe. You don't always realize that they are upstairs, because the timbre sounds like the rest of their horns, and they work their way up and back down before they make Fido start howling.
My advice (which I'm sure will not be heeded) is that if you are an alto player, don't ever go above a high B- EVER! I know many of you think it's cool to cause sonic pain, like a Harley-Davidson rider at an AARP convention, but just because you can do it doesn't mean you should.
I do have my students practice overtones, to gain more control over their upper oral tract. Overtones will definitely help gain more control over your altissimo range, since altissimo note are really just overtone notes. Mastering overtones (not necessarily to the Sigard Rascher extreme) will also help you gain control of your timbre, meaning you will be able to control the upper partials in your sound. Being able to shade your sound consciously is a wonderful skill.
There are many different fingerings for every altissimo note and the alto fingerings are mostly different from tenor fingering. Each brand of horn is unique with regards to which fingering work the best. You should experiment with many different fingering charts until you find the best fingerings for your horn. Even though a particular fingering might sound the clearest and be the most in tune, it may not be practical because you may not be able to approach it smoothly from the fingerings of nearby notes. You need to work out the fingerings that you can play smooth scales with. Get out your tuner because the pitch will be your primary consideration for choosing a fingering, technical considerations will be a close second, and lastly will be the timbre of the fingerings.
Before you embark on this high note exploration though, have mercy on Fido and buy him some earplugs.
Top-Tones for the Saxophone: Four-Octave Range
Overtone exercise I got from Joe Viola
Flavio's alto fingering chart
Altissimo fingering charts by saxophone make
Lower altissimo chart
Middle altissimo chart
Upper altissimo chart (Danger: Don't try this at home)
Jason Dumars' favorite altissimo fingerings