I would hit them with FIM when the nodes start alternating. Once you get alternating nodes, you will have multiple tips as well, and can really go to town. Not only does this ensure you have a healthy enough plant to FIM, but it allows you to do it rapid fire around your plant (not all at once of couse) because you have tips ready to FIM while the previously FIMed tips are healing. I've seen some funky looking plants that have been FIMed multiple times while also under LST that turned out looking like 8 plants growing from a single stem. The yield was damn near what you would expect from multiple plants too...