The only problem I have encountered in using the description field in Ancestry.com or FTM2012 is that the description field and the place field get merged into a single place field in gedcom transfers to other software. It becomes a huge mess trying to clean up the locations once that occurs.
If you plan on using FTM and Ancestry alone and aren't worried about being able to share the tree with someone using another software, then no problem.
If you create custom events, be careful about the label you use for the event. If you inadvertently use a label already in use by GEDCOM standard or software specific labels, it may cause problems duing gedcom tranfers later on.