HI chah
For 1LAC/BSC you have some benefits in term of moderated signaling on BSCs, A interface & MSC. SO it is recommended in first network planning.
BUT so after, according to your LA optimization & BSC capacity you can use LAC/different BSC, & furthermore you can use LAC/diff MSC but seldom.& it increase the paging signaling on your BSCs, MSCs & A links, so you must balance between LU events, & the the paging load followed your MSC/BSC/LA configuration.