Wasei-eigo and most Gairaigo (especially in a text or sentence as opposed to being by itself) is usually written in Katakana (イメージ, ジュース, スマート,パンツ,アベック). However, there are times that have seen some in their original Romaji form (image, juice, smart, pants, avec) even in a Japanese sentence, ex. 彼女のpantsです; juiceが好き; ジョーンはsmartですよ; 数々のavecを見た; etc.
Is this common or incorrect? What does it mean if you do this as opposed to using Katakana?