I think it is not reliable, because formalin has an osmotic pressure of about 32 atmospheres (almost 5 times more than that of mammalian blood) which may affect the tissue weight. Nevertheless, prolonged fixation in formalin may show a decline in the tissue weight, perhaps because of the tissue shrinkage due to the loss of tissue fluids.
I have attached a research link for your better understanding.
Article Effect of storage in formalin on organ weights of rabbits
As Mr. Debasish Kumar Dey says, it is not accurate and reliable, but at least you can compare the intervention group with control group, only if you measure the weight of both groups under the same condition (formalin fixed VS. formalin fixed).
After all, it depends on your purpose.
Of course, needless to say, it is the best to measure it before fixation.
If it is just for statistical comparison there is no problem. If your goal is to give weight+- standard error results, it's not fair. You can weigh other organs before fixation (exp n=6) and after to get an idea of the increase in weight caused by your fixative solution. Determining a coefficient will greatly help you to know the real weight before fixation. You must use the same tissue.