If you normally commute down from Pike's Peak to Colorado Springs, then you should be nervous. Otherwise, no, it's pretty rare to fill up the battery. It's sized so this won't happen often. Otherwise you would waste any further regenerative braking opportunities until you had a chance to use up the energy in the battery to make room for more.
Strangely enough, I have never been to Pikes Peak -- even though I have driven cross country 5 times.
But anyway, never full though? What you say, makes sense, have enough capacity to capture all the "free" energy you can. No use creating it, and not being able to store it. But conversly, if in 1000 miles, I never see it at all -- wouldn't that mean I am carrying excess capacity? and thus extra weight?? Obviously it is one of many lines the engineers had to draw, and I don't mean to bash it, just explore it.
I have really tried to hit the full mark. Even shifting to 'B' when going down hill as much as possible. Setting the cruise control for just the right speed so the engine is running and charging. Just to see it get full. Just for Ha Ha's; can't do it.
Toyota managed to accomplish a 100,000-mile guarantee (and a 200K or 250K expectation) out of NiMH batteries by writing a computer program that would hold the battery between about 25% (?) and 80% (?) of its capacity. The battery will NEVER be full, and barring a failure of the ICE will never be empty.
The battery indicator you see on the MFD only covers the above range, so if you did see 8 bars full, your battery would still be only at about 80% (?) of capacity. And the computer even tries to avoid those extremes. The reason you almost never see 8 bars is that the computer is working very hard to hold it below that.
The extra weight you are carrying by having too big a battery is the reason you probably will never have to replace the battery in the life of the car. Full charging and deep discharging shortens the life of NiMH.
If you've only got 900 miles on your Prius, I'm pretty sure it wasn't the car you took cross country 5 times.
Anyway, you haven't described the area you normally drive in. If it's mostly flat, you aren't the customer that Toyota drew the line for. You might be carrying excess capacity for you, but that same capacity might not be sufficient for other customers. Toyota only makes one model so far, so it has to be "one size fits all" even if its a bit baggy on some and a bit binding on others.
To once again answer the question: No, you should not be nervous that you haven't seen a full battery meter. After 88K miles, I can count on 1 hand the number of times I've seen it. The Prius battery control system tries to move toward ~60% full whenever it can reasonably do so. The farther away it is, the harder it tries.
Hrm. I've seen mine at 'full' (and noticed it use engine braking while coasting because it won't regenerate any more,) a few times, mostly after coming down a couple local long freeway hills that you can coast down and hold an even 60 mph without touching the gas or brake. I've only seen it down to one bar once, and that was when I sat in a parking lot with the A/C and radio running for a long time, eating lunch.