How to deal with deprecated functions?

[Marko Käning]

Suppose you've got some port which throws during build "deprecated functions" warnings:

1) How critical is the use of these functions actually? (As long as the compiler throws warnings they are still usable...)

2) When should they latest be replaced by the recommended functions? (Suppose there are some deprecated in 10.5 although 10.6 is current.)

3) How many pre 10.5 systems are actually still out there?

Any recommendations how to deal with this?