Okat first that is not FIMming. That's just pruning.

You should NEVER FIM or top or pinch in flower under any circumstances~ it confuses and stressed that plant and WILL severely hurt your yield. There's no 'might' about it.

However, pruning the weakest low branches is fine. Only remove thin, spindly, weak low growth. This actually improves the % of your yield that is prime dense tops. The yield will be the same, but it will be much nicer, and you are also less likely to have mold problems with the improved air flow from taking out scruffy crap below the canopy.

For an extreme example of this, Google 'pistilwhipt method'.

EDIT: Josht please don't try to give advice on subjects you are not familiar with, thanks. It ends up being really confusing, and someone who actually TAKES that advice could lose their crop... NOT cool.