Some writers are prone to append peripheral documents of various kinds to their proposals on the theory that the bulk will buttress their case. Reviewers almost never read such appendices, and may resent the padding. The best rule of thumb is: When in doubt, leave it out.
Appendices to proposals are occasionally used for letters of endorsement or promises of participation, biographical data sheets (when there are too many--say, eight or more--to be conveniently placed in the "personnel" section), and reprints of relevant articles.
If two or more appendices are included in a proposal, they should be designated Appendix A, Appendix B, etc.