This is what I dug up with the help of a member :
GENERAL RECOMMENDATIONS FOR UNDERLAYMENT
To assist the contractor, we summarize our ideas for underlayment placement and orientation, panel end, edge, and perimeter gaps, and nailing. The recommendations given for nailing are more conservative than specified in ANSI A108-1999, Section AN-220.127.116.11, which states, “locate nails at 6-inch centers along panel edges and 8-inch centers each way throughout the panel…" The closer nail spacing in Table 1 will better guard against voids between the subfloor and underlayment sheathing layers, improve the composite action of the two layers of sheathing thus reducing sheathing curvature under service loads, and it will increase the buckling resistance of the underlayment thereby minimizing the potential for buckling of the underlayment due to seasonal moisture content changes.
Place underlayment panels (Exposure 1, plugged-face plywood of minimum 3/8˝ thickness) such that the following conditions are met:
1. Butt all underlayment end joints at 1/4-points between joists as depicted in Figure 3. Example: Butt underlayment panels on either side of the joist centerline at: 4˝ for 16˝ o.c. joists, 5˝ for 19.2˝ o.c. joists, or 6˝ for 24˝ o.c. joists. Underlayment end joints should be placed as far away from subfloor end joints as possible.
2. Underlayment to overlap edge joints of subfloor by 1/2 of the width of the subfloor panel (24˝). At restraining surfaces, overlap may be less than the 24˝ when the subfloor panel is less than 48˝ wide.
3. Gap underlayment panels 1/8˝ on all ends and edges, and 1/4˝ at perimeter walls, cabinetry, or other restraining surfaces.
Description of the type of plywood underlayment and the recommended fastening schedule are given in Table 1.
The following guidelines are recommended when fastening underlayment panels.
1. Use ring-shank nails or screws (no drywall screws).
2. Fasteners should pass through entire thickness of underlayment and subfloor panels with minimal or no penetration into joists.
Here's the link to the complete article : http://www.schluter.com/5138.aspx