30,000ft is not a usable altitude in most airspace.

Flights flying up to 29,000ft are separated by 1000ft with even altitudes being Westbound flights and odd altitudes being Eastbound. Above 29,000ft the spacing shifts to 2000ft increments so the next useable altitude above 29,000ft is 31,000ft which is Westbound altitude.

There is a new system called Reduced Vertical Separation Minimum (RVSM) which is in effect in certain oceanic airspace and will eventually be phased in for domestic airspace. RVSM uses 1000' increments both above and below 29,000ft. In order for an aircraft to operate in RVSM airspace it must have upgraded RVSM equipment (altimeters, pitot/static system, etc.) and the airline must be authorized for RVSM operations. It is the conversion of the fleet to RVSM specs which is slowing the transition to RVSM in the domestic airspace.

Once you've determined the legal altitudes for your flight you find the optimum altitude for your flight.

Jet engines are more efficient at colder temperatures so, in general, you want to go as high as possible until you reach the point where the temperature stops dropping with increased altitude (tropopause) which typically occurs at roughly 36000ft (though that can vary considerably with season and latitude). Another factor is absolute temperature (vs relative temp) and aircraft weight. A very heavy airplane, in a warmer than normal atmosphere, may not be able to reach the coldest altitude so it's optimal altitude may be lower.

After all of that the last big variable is winds aloft. The wind speed and direction can vary considerably with altitude so it may be more efficient to pick a less efficient (warmer) altitude in order to take advantage of more favorable (or avoid less favorable) winds.

Oh, one more... Turbulance. The more efficient altitudes may have a rougher ride so a less efficient altitude may be choosen for passenger comfort.