BathroomsYour bathrooms will be the smallest rooms in your house, but you'll almost certainly want to have a few of them, especially if you plan on throwing parties. Every guest or resident will need to use the bathroom once or twice a day; when you have a party, though, everyone will want to hit the bathroom after they eat, so you will have quite a few people trying to get into them all at once. Thus, you're going to want to have them easily accessible.
At its most basic, a bathroom is a simple 2x2 room with a toilet and a sink. Even the sink isn't absolutely necessary if there's one available somewhere nearby, such as in the kitchen. If you want to just have simple stalls, either via the ResiStall Astro Divider or by building up a 1x2 room around a toilet, feel free to do so, but remember that your guests will usually want to wash their hands after they do their business, so give them some means of doing so.
So far as toilets go, there are only three to choose from at this point. Either of the more expensive options is better than the Sewage Brothers cheapy model, which is less reliable and offers no comfort. The ResiStall offers the same Need benefits as the Mentionable toilet--both give you a large bonus to Comfort while your Sim is sitting--but the ResiStall offers built-in privacy. Remember that most Sims won't use the bathroom unless they're not within eyesight of another Sim; a ResiStall will solve this problem, but it does require a lot of space to install, being 1x3 while the other toilets are 1x1. If you have enough space for a large communal bathroom near your kitchen area, though, a line of ResiStalls may be just the solution!
Unfortunately, the Sims 2 does not as yet have one of the nifty sonic showers that were available in some versions of the previous game, so your Sims will have to resort to the low-tech liquid versions. For the most part, showers are better to take than baths, if only because they're quicker. Baths will restore some of your Comfort need, but again, this is rarely something that will drop critically low if you've invested in a decent bed and some good seating for your dining room and living room. If you have the scratch, the Colonial ComboCleen is the best bathing bet for the money, as it will let you choose to take a shower or bath. Toddlers can only be bathed by a grownup in a functional bathtub, so it's worth getting one of these combo models if you plan on having kids. (Newborn babies can only be bathed in a sink.)
You don't need to worry about putting a shower or bath in every bathroom; only those close to your main bedrooms should have them. Guests won't ever satisfy their hygiene requirement via bathing, unless you invite them to spend the night, so placing bathtubs in the common bathrooms will just waste space.
Bedrooms are the simplest rooms to design; all you really need is a bed! Your Sim won't be spending much time here, so feel free to splurge on an expensive bed, but you won't need much else in one of these, save perhaps a dresser. If you plan on Woohooing a lot in your bed, though, you might want to boost your Environment score here a bit with some decorative items. Beyond that specific act, though, you won't have to worry about your Environment score here, or even your Mood at all, since you'll normally only be sleeping in your bedroom. If your Sim is sleeping alone, and you want to have an efficient way to get them through the morning routine, you can get them a shower and toilet and plop them down right into the bedroom; you'll just need to be careful about making messes.