A 9x9 mm WLCSP?  An underfill or a cornerbond could damage the thermal cycle performance of the package.  As is, without underfill or adhesive, at 9x9 mm in size, the board level thermal cycle performance won't be great anyway.

If you prefer to use a cornerbond, then consider a reworkable edgebond adhesive.  Traditional edgebond adhesives, with low Tg and high CTE, will result in poor WLCSP thermal cycle performance.  A high reliability reworkable edgebond adhesive will enhance WLCSP thermal cycle performance.  At the upcoming ECTC conference, there will be a paper co-authored by folks from Cisco, Zymet, and Portland State University that presents data demonstrating exactly that.  The 8x8-mm WLCSP used in the work is a bit smaller than your 9x9 mm package.

If you prefer underfill, then Zymet has high reliability reworkable underfills with low CTE's.  (I am at Zymet)

Karl Loh