Would it be possible to not have default values in the field 'issue type' so this appear blank and the requester has to select the appropriate one from the dropdown each time they create a new request?

[Nic Brough -Adaptavist-]

Yes.  Go to the issue type scheme for the project and remove the default issue type.

Bear in mind that JIRA will remember what project and issue type the user last created and use that once a user has made an initial selection, but it will forget it after a while, or after the user goes through something that means it can't be sure what project they're in any more.

[Samidurai Gopalan]

Egarly Awaiting for replys