If you want compatibility with the overwhelming majority of existing Linux installations, go with libc5. Distributions installing glibc2 also install libc5, so you should have no problems. There are two basic reasons not to port to glibc2. Glibc2 is still being developed, and ISV reviews of its use have been mixed. Glibc2 is also closely linked to kernel releases and has not proved stable enough to be universally adopted.
Most people seem content to wait until things mature.
Porting to glibc2 excludes your application from a very large installed user base of libc5 systems. Though some argue that by providing source code to the application allows the binary to be recompiled when the libraries change is, in fact, impractical. A potential customer, say a large bank, will be both unwilling and unable to recompile and re-install the code on several hundred workstations. You don't need an MBA to know that this is not a winning sales pitch. The problem of keeping your application maintainable is addressed by every ISV port.