It depends on your memory controller application. Some DQS pad groups may not provide enough number of associated DQ pads because some DQ pads may not be bonded out. Although not many DQS groups have difference DQ sizes, you should pay careful attention to choose a DQS pin and check if the all associated DQ pads for the selected DQS group are enough to meet your application need.
The decision will usually depend on whether your memory controller uses eight DQ pads or four per DQS. While majority DDR memory applications require eight DQ pads per DQS, there are some others like a RDIMM memory controller that uses only four DQ pads per DQS.
To know how many DQ pads you need per DQS for eight DQ per DQS, use the following guideline:
Minimum number of DDR1 DQS group pads:
DQS (1) + DQ (8) + DM (1) = 10 pads
Minimum number of DDR2 DQS group pads:
DQS (1) + DQ (8) + DM (1) = 10 pads (single ended DQS)
DQS (2) + DQ (8) + DM (1) = 11 pads (differential DQS)
Minimum number of DDR3 DQS group pads:
DQS (2) + DQ (8) + DM (1) = 11 pads (differential DQS)
Note:
1. This guideline considers DM as a mandatory signal. If DM is not required, you can subtract one from the minimum required size.
2. If a DQS group includes a VREF1 pad for the bank, you have to count one additional DQ/VREF1 dual function pad.