What do you mean under "signature"?
1. If .Net Metadata binary representation (in metadata Methods table) - in that case signature of method will be untouched, even if method obfuscated.
2. If you mean full name of method (for example int MyMethod(int firstParameter, out bool secondParameter)), if you apply [Obfuscate(Exclude=true, ApplyToMembers = true)] to its declaring class, in that case this method will excluded from obfuscation and its full name will be untouched (excluding the case when method implementing/overriding method-ancestor from obfuscated class-ancestor/interface, if method-ancestor is obfuscated)