With previous el5 and el6 installs, I would clear the disks, and accept the default lvm layout, though I would adjust the sizes a bit. Ordinarily when I adjusted the sizes, I would leave myself additional free space within the volume group so that I could grow later as needed.
Upon installing the rhel7 beta, I discovered this is no longer how disk allocation works. After you select your disk(s) and adjust the sizes of the logical volumes how you wish, anaconda creates partitions just large enough to cover the space you assigned. To the end user, this means that immediately following installation the volume group is 100% full, with no free space immediately visible. To use the rest of the free space on the disk, you must create more partitions, initialize the new partitions as physical volumes, and add them to the volume group. Only then will you be able to use your remaining free space.