You can top/FIM when your plant has at least 2 nodes and can top/FIM when you are up to 5 nodes. After that, some continue to top/FIM a few more times on the new sites they have created. Plants/strains react differently, some don't mind it all and others show signs of a little stress. In most cases, it will slow the growth and add time to the cycle.
I'm sure others will have other opinions, this is just what works for me.
Welcome, good luck and keep us posted!