It is recommended to rebase before delivering, and it usually is based on the "recommended baseline" mechanism.
So even if you said "irrespective of baseline is recommended or not", I would still suggest you look at "this deliver policy: 'Rebase before delivery'".
This policy (Require development stream to be based on the project's recommended baseline(s) prior to delivery
) requires developers to rebase their source streams to the target stream current recommended baselines before they deliver work to the target stream.
The policy can be set per project or per stream, for interproject and intraproject deliver operations.
The goal of this policy is to have developers build and test their work in their development work areas against the work included in the most recent stable baselines before they deliver to the target stream. This practice minimizes the amount of merging that developers must do when they perform deliver operations.
From UCM policies:

If you don't recommend baselines on the parent stream, then you need to define a preop deliver trigger (as in this thread):
cleartool mktrtype -ucm -preop deliver_start -execwin ccperl "\\share\path\to\script.pl" -c "Check if rebase activity exists before delivering" rebase_before_delivery@\pvob
With script.pl
a script which would list all activities of the current stream, checking if the latest activity is a rebase.xxx
one.
http://www.ibm.com/developerworks/forums/message.jspa?messageID=14116547