See http://www.ibiblio.org/apollo/Documents/tn_dNASA Technical Note D-5869_1970023342.pdf5869. This document includes a discussion about choosing the location of the navigation instruments of the Saturn V rocket. See the figures on pages 11 and 13 and nearby text (page 12).
Several considerations were mentioned, including the effects of bending modes, and other environmental factors of possible locations, length of cabling needed, and so on.
The bending mode shapes show the magnitude of the amplification of lateral motion. You would prefer not to place motion sensors at a position that bends (a lot) at a similar frequency to control motions of the rocket, as the bending could feed back into the control loop and lead to instability.