There is nothing wrong with c-tree temp tables. They are faster. SQL Temp tables take a bit of work to create so ctree has less overhead.

The only reasons to change them back to Default or SQL are:

1) You are using them on a report. This allows SQL optimization when generating the report with a single SQL Joined query.
2) You want to use SQL optimized functions such as range where clauses or range copy.
3) The number of records being placed in the temp table is huge and it could cause a local workstation’s hard disk to fill up.

Otherwise, just leave them as is. My policy is to use ctree unless one of the points above says that SQL would be better.