From my little document, I found some info on this & I think it may useful to continue on this topic
The immediate assignment on TCH is a good way to use the resources more efficiently.
We have two kinds.
1) TCH first (assign signaling on TCH regardless of SDCCH congested or not). & it is not recommended strategy.
2) Other strategy is SDCCH first, which mean when all SDCCH are occupied, the coming call setup will be accomplished on TCH instead, to get a minimum trunking loss. So there is a balance between signaling load added on TCH & the extra capacity can be added on TCH by reducing TS reserved for SDCCH,
Also there is a different recommended value for SDCCH dimensioning when using direct assignment on TCH.