IME yes, they do account for that. Now that said, there are definitely things that are only good for two trimesters, such as non-stretchy tops. A lot depends on how you carry as well--taller people (like you?) might be able to go longer, but my torso is so short that I stick out very far at the end. THAT SAID I have learned that a few pieces that fit well in the second but not third trimester are GREAT for postpartum, especially tops that have easy nursing access. I like (actually I love) this top that I have and I know it will work for that: http://www.gap.com/browse/product.do...&pid=926403002
Actually, any of the Gap "boyfriend" half button tops will be great for this.
Length is really crucial at the end. You may try on something now and think it's a dress but by the end of pregnancy it will be the right length and you'll be happy you have it. Nothing like the bottom of your hanging down stomach peeking out under a shirt in winter.
For very inexpensive, cute basics I like H&M mama. I do size up there, though. They ship and it's cheap shipping as well. Old Navy hasn't been great for me. I would not buy pants there. Shirts, meh, if you like it then do it. Returns are easy. Gap is overpriced and the fit can vary, but if you want to wear things that don't scream "pregnant lady clothes" it's a good store. But I normally wear my pre-preg size there. They have amazing sales (especially in store) every 3 months (early August I found some stuff for almost no money, and looking forward to early November). They always have something on sale, too.
Style: for me, I don't like the tie under the boobs look. It just doesn't do much for my short torso, and the empire waist top makes me unsure of what to do on the bottom. So I avoid it. I like ruched sides. Allows for a good fit that a little more sophisticated, if that matters.
Jeans, I think you get what you pay for. I avoid Motherhood stuff whenever possible because I don't like how it stretches and washes. I think Gap jeans are fine and I also like my designer jeans that I bought used from Ebay for less than the cost of Gap. Cheaper brands like Old Navy definitely bag out in the butt and behind the knees in an hour, so buy tight if you buy there. I LOVE full panel because I think the demi panel digs in during the third trimester, but I know that people hate them and feel like they're so ugly. But nobody sees it, and I feel like it gives me a smoother line under shirts, too.
You may consider some support garments as well if your back starts giving you trouble.
I know I have written too much and could write more.