Weighing yourself before or after exercising
Personally, I weight myself in the morning after I wake up. Once or twice I weighed myself before/after exercising but it wasn't very healthy for me because I need exercise to be about health and habit not just weight loss. When I weighed myself after exercise and didn't see a change in made me a little annoyed. I realize now that it works best if I do it in the morning and don't vary the time of day. For me, I guess I have to be careful that I don't get too obsessed with the numbers so that's been what works best for me. Good luck. :)
I try to stick to early am (also after BM and buck nekkid) on days where I didn't exercise long the day before. For example, I did a long run of 12 miles yesterday and today's scale registered a "gain" of 1 lb. Not really, since it's probably just muscles & joints retaining water and it'll all pass. But a PP gave a good suggestion of before and after - NOT for your real weight but for comparison of how you need to hydrate.
First ultra: Stone Mill 50 miler 11/15/14 13:44:38, First Full Marathon: Marine Corps 10/27/13 4:57:11, Half Marathon PR 2:04:43 at Shamrock VA Beach Half-Marathon, 12/2/12 First Half-Marathon 2:32:47, 5K PR Run Under the Lights 5K 27:23 on 11/23/13, 10K PR 52:53 Pike's Peek 10K 4/21/13, (1st timed run) Accumen 8K 51:09 10/14/12.
I do step on the gym scale before my work out; that way when I work out especially hard I can tell how much fluid I need to replace. I almost always lose a pound in a workout but have had 2, 3 and even a 4 pound loss on especially grueling days. Other than that, I don't have a scale at home and don't plan to get one until I am close to or at goal.