On 8GiB disk image I did:

clearpart --all --initlabel
part /boot --fstype=ext4 --size=500
part pv.253002 --grow --size=1
volgroup vg_root --pesize=1024 --reserved-space=2048 pv.253002
logvol / --fstype=ext4 --name=lv_root --vgname=vg_root --grow --size=1024
logvol swap --name=lv_swap --vgname=vg_root --size=1024


# vgs
  VG      #PV #LV #SN Attr   VSize VFree
  vg_root   1   2   0 wz--n- 7,51g 2,00g


# vgdisplay vg_root | grep Size
  VG Size               7,51 GiB
  PE Size               1,00 MiB
  Alloc PE / Size       5642 / 5,51 GiB
  Free  PE / Size       2048 / 2,00 GiB

# lvs
  LV      VG      Attr     LSize Pool Origin Data%  Move Log Copy%  Convert
  lv_root vg_root -wi-ao-- 4,51g                                           
  lv_swap vg_root -wi-ao-- 1,00g 



Sizes match expectations. 


Then I did:

clearpart --all --initlabel
part /boot --fstype=ext4 --size=500
part pv.253002 --grow --size=1
volgroup vg_root --pesize=1024 --reserved-percent=10 pv.253002
logvol / --fstype=ext4 --name=lv_root --vgname=vg_root --grow --size=1024
logvol swap --name=lv_swap --vgname=vg_root --size=1024


# vgs
  VG      #PV #LV #SN Attr   VSize VFree  
  vg_root   1   2   0 wz--n- 7,51g 769,00m

# vgdisplay vg_root | grep Size
  VG Size               7,51 GiB
  PE Size               1,00 MiB
  Alloc PE / Size       6921 / 6,76 GiB
  Free  PE / Size       769 / 769,00 MiB

# lvs
  LV      VG      Attr     LSize Pool Origin Data%  Move Log Copy%  Convert
  lv_root vg_root -wi-ao-- 5,76g                                           
  lv_swap vg_root -wi-ao-- 1,00g         


Moving to VERIFIED.

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
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    New Contents:
Change: Two new options were added to the kickstart volgroup command to specify initially unused space in megabytes or as a percentage of the total volume group size. These options are only valid for volume groups being created during installation.

Result: Users can effectively reserve space in a new volume group for snapshots while still using the --grow option for logical volumes within the same volume group.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2012-0882.html