This is a MUST bump thread. The information on Styleforum has been extremely valuable, literally, when altering clothing, but I think someone needs to explicitly say "don't do it" pertaining to this topic.
"Taking in vs Letting out" pants, especially anything non wool, such as cotton for example, should never be "let-out" in the waist under any circumstances. I'll save everyone the money and say, don't do it, despite the recommendation of a few on here, that it's ok. It's not.
I just had a $300 + pair of khaki cotton trousers, brand new, with tags (Loro Piana/Zegna) waist taken out about 1 inch. A fairly standard refitting. As many on here suggest -- go for the better seat and leg fit, I did just that. I'm blown away by the damage left from just a little adjustment. There are noticeable white lines (in the form of a V) up the seat to the waist and deep white marks where the waist was let out. The tailor told me that this happens with ALL cotton, really anything, but wool and even then, it's not always perfect. Its crazy that a manufacturer -- especially one of such high quality would not dye the pants then stitch or whatever they do. Why include 2 inches of extra fabric in the seat if it's entirely unusable? Bizarre. My tailor (a good one here in NYC) even told me not to do it. It was my choice and not a result of the tailors work. I wanted to see for myself. I wasn't 100% sure that it wouldn't work based on what I read on the internet.
If you come across this and it pertains to the "waist," always go the size up (or the right size) and let the tailor take in the waist an inch, seat and slim down the leg. Tailor says this is the only way to go! I've done it before with great results. I had never let the waist out of a pair of pants. This time I tried to save a little money getting what I thought was a better fit and less tailoring and it's going to wind up costing me 3 times what I originally wanted to spend. Clothes are fickle and nothing like buying a TV. Lesson learned. Fortunately the pants were marked down and the tailor was reasonable with cost.