Is it okay to measure flour by weight by converting from volume?
Short answer: YES.  Measuring by weight is actually the better way to measure flour.

In fact, measuring flour by weight is the preferred method of measuring it in most places.  This is because while measuring by volume is more convenient*, measuring by weight is more accurate.  If you weigh your flour, however, you will always know you are using the same amount, whereas two individually measured cups of flour (done by volume) can have wildly different weights.  This makes recipes where you measure by volume much harder to repeat reliably and perfect with small tweaks.

For a more detailed reference on converting cups of flour to grams, see this answer. As noted in the answers to that question and in the comments below, the average weight of a cup of flour can vary greatly, between 4 and 5 ounces (about 110 to 140 grams), though a heavily packed cup could weigh much more. I personally use 1 cup = 4.5 ounces (125 grams) when I need to convert from volume to weight, but your results may vary depending on the recipe you're using  



*There seems to be some contention here.  I'm from the US, where most people don't have kitchen scales (I'm the only one I know who does, despite being friends with lots of foodies).  For us, measuring by volume is practically mandatory, and if I want to measure something by weight, I have to convert the measures myself since they're usually given as volume only.  In other parts of the world, the situation is exactly the opposite -- everyone has scales, cups are nowhere to be found.  Convenience is obviously relative.