Firing the bisque and glaze can be tricky and a lot depends on the qualities of the clay and glaze, as you have discovered.
There are optimal temperatures for both the glaze and the clay and the relative differences will control certain qualities.
For the clay - as was mentioned - there is a maximum high temperature for each clay. After that the clay will bloat, deform and eventually melt. When you buy the clay you should be told the maximum temperature. This is not always the case, however and you may have to discover it by over-firing.
Lower than the maximum, the temp you fire to is personal choice - you may fire to different temps to get different amounts of vitreousness, surface color, surface texture, to fit a particular glaze, etc. For functional pottery, for example, it is important that the clay is vitreous, ie. the clay will not absorb water and your vases will not leak all over the antique wood table, etc.
For the glaze, the same is true. There is a maximum after which the glaze will run down the sides of the pottery or bead and get all over the kiln. There is a minimum at which the glaze will not get melted and look good.
Then there is the "fit" of the glaze to the clay. They have to be compatible at the temperature at which you want to fire the clay. For example, there is the case of crackling or crazing because the clay shrinks more than the glaze can handle. This can be avoided in some cases by firing to a lower temperature. Or, like was mentioned, if the clay cools too fast it can cause crackling.
I would also like to mention that there can be an optimal bisque temperature in some cases. The clay will shrink in the bisque and also in the glaze firing. Sometimes the glaze fit will depend on the bisque temperature. For example, if the bisque is at cone 010 the glaze may craze or crackle or shiver but it is fine if the bisque is at cone 04.
As far as soaking, I also do not believe there is a benefit to soaking a bisque kiln, however, as was mentioned, there appears to be a benefit to the clay body in some cases by firing twice. Glazes can certainly be affected by the rates of firing and cooling and the amount that the kiln is "soaked" or kept at a certain temperature for a while. You will have to experiment to see how your particular glazes is affected.
So, hope that rambling is helpful.