If you have a base glyph that doesn’t fit well with top accents (think of a g with an ear, for example), you can create, for example, g.topAccent. When automatically creating the composites for gcaron, gcircumflex etc., Glyphs will prefer the g.topAccent.
Annoying behaviour, by the way: if I have a g.topAccent and a g.ss02, a gcircumflex.ss02 will still be built with g.topAccent. Could this be changed? A g.topAccent.ss02 or similar should be required for that.