You can use whatever you want as long as you configure it consistently and correctly. The technology prefix was designed for H.323 systems where you may want to choose varying gateways depending on the call type you want to make. The most common example of this was perhaps 9# for voice calls using a voice gateway and 8# for video calls using an H.320 gateway. You typically had separate gateways for these scenarios. If the user fails to enter one, the default technology prefix is what is used. That way the gatekeeper knows which gateway(s) are appropriate in the zone for that call.
Cisco IOS H.323 Configuration Guide, Release 12.4T
http://www.cisco.com/en/US/docs/ios/voice/h323/configuration/guide/12_4t/vh_12_4t_book.html
