Your question really has two parts:
Must all the figures start and end at the same altitde at which the roll was executed? No.
Must figures such as hammerheads, humpty bumps, and half cubans exit at the same altitude at which they were started? No.

However, bear in mind that all figures start and finish in a wings level horizontal attitude. As such, the exit altitude of one figure determines the entry altitude of the following manuever. For example, a loss / gain in altitude after completion of the roll is deducted from the score of the next figure at 1 pt per 10 degrees. Hope this covers what you were looking for.
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