Should I use occasional feminine in order not to sound old-fashioned?

Well, if you opt for the Bergen dialect/accent, you can avoid the feminine gender altogether. However, even in (West) Oslo, people who use the common gender most of the times, prefer the feminine gender with definite forms of some nouns, in order not to sound too conservative, for example:  katta f. ''the cat'', kua f. ''the cow'', jenta f. ''the girl'', hytta f. ''the hut'', øya f ''the island''.