1. Pre-baby. A shower is a shower. When pregnant, it bugged me not to take very hot showers, which I prefer. (Lots of heat is supposed to be bad for the baby.) I am a homebody and a slob, so my usual shower routine is Day 1: shower; Day 2: no shower; Day 3: shower (but only if I have to leave the house). The upshot of which is I'm not the sort of person whose nose is going to be out of joint over two or three days without a shower, nor am I the sort of person who would generally choose to shower before, say, watching another episode of not-terribly good TV.
2. Newborn. Suddenly a shower was the holy grail, especially once Andy went back to work after two weeks. He would come home from work and I would head up for a shower, and it was untold bliss. It wasn't just that the shower itself was pleasant. It wasn't just that it was time alone. It was that if the baby was crying, not only was it not my problem, but I couldn't even hear it. Bliss! I still remember the sensation of turning the water off and listening for crying, and the relief when there was silence.
Some babies like being in the room while you're showering. They like the white noise and, apparently, the steam. Not Ben. I tried it once or twice, putting him in his laundry-basket bassinet or his bouncy seat, and he shrieked his baby head off. Oooooooh-kay, then.
3. Settled Baby. Now that the kid was napping reliably, I could put him down and then take the monitor with me into the bathroom. The trouble was, his nap time was my only freedom, and I hated to waste it on showering when I could be watching mediocre television or reading the Intertubes. Once he was crawling, I tried bringing him into the bathroom again, this time unrestrained, with all hatches battened of course, and an armload of toys. No dice. He'd be fine for a little while and then he'd get upset. I think it was the steam that bugged him. Mean mommy that I am, I still did it to him about half the time, alternating with naptime showers.
4. Toddler. When Ben was around 12 months and her son was around 18 months, I asked my friend S how she handled showering. She said she left him running around free while she showered. That had not occurred to me. I don't think I moved to free-range showering for a while -- certainly not until he was a well established walker -- but that was where we ended up and where we've stayed. He has access to the bathroom, his bedroom, and the upstairs hallway that connects them. If he wants me, he can come get me, and he can't get far enough away that I can't hear him if he's yelling.
At first he found that upsetting, too. He'd come stand by the tub and scream and cry for a minute or two, then get distracted by some mischief and disappear. It took maybe half a dozen iterations before he was totally fine with the routine. One time I heard his door slam and so had to leap out sopping wet and rescue him from his room. After that, I learned to block all doors. He went through a (mercifully brief) phase of bringing books and toys and trying to chuck them into the shower with me. He went through a (long, but now sadly over) phase of running down the hall as soon as the he heard the water stop to come stand in the doorway and applaud.