3GPP has already mentioned QCI values for each different kinds of bearers. Each bearer is defined to fulfill a goal.
Before IMS SIP signalling message exchanges, UE ask network to create a pipe so that it can exchange SIP signalling with P-CSCF node. To do so, UE sends PDN connectivity request to MME for IMS APN.
When MME receives PDN connectivity request message from the UE, first it checks whether UE subscribed for IMS service or not.
Let's assume UE has IMS subscription, now MME initiates Create Session Procedure towards other network nodes and it asks nodes to establish default bearer with QCI "5" similarly the default bearers if any established for other APNs.
As far as I went through specifications and have LTE knowledge, I always seen QCI "5" bearer for SIP signalling.