Absolutely, yes.
Especially so if they are planning a flight fully-loaded, where even a mere few pounds can make the critical difference between a safe takeoff and a crash.
Okay, I've gotta disagree just a bit here.
If a few pounds makes the difference, the "safe" takeoff was on the bleeding edge and I will say that it doesn't fit my definition of safe.
*Maintenance* is not going to make a difference in W&B unless something was replaced with an unlike part. This usually involves equipment upgrades, STC's, etc. "Maintenance" means, take out broken part and replace with identical variety of part in non-broken form.
I can see how a school with 30 172's might make this a little bit of an issue, but do the students actually fly all of them? It seems to me that most students settle on a "favorite" plane and fly that one as much as they can. If they're flying that plane, and they just flew it a couple days ago, and they're on an instructional flight with well-known parameters, and they're nowhere near the limits - I think it's a little bit much to say "you have to do the EXACT weight and balance, again."
I will not discuss this any further. Safety comes first, and you cannot rationalize it away. Rationalization at the expense of safety, to create a convenient situation, should be listed among the hazardous attitudes listed in the FAA flight-training handbooks (PHAK and IFH, for starters). I am sure it falls under the category of "impulsivity" in the list of the 5 attitudes. Any PIC who gets into a crash, and actually manages to survive, and later is found to have not done a proper weight and balance, should have the book thrown at them and criminal charges, including negligence, filed.
Actually, I will state that being as anal as you are (sorry, that's how I see it) about weight and balance may cause the student to be LESS safe.
Hear me out here: Two scenarios.
Scenario 1: CFI teaches pilot about weight and balance, and has pilot calculate weight and balance in a particular airplane for several different scenarios, and plot the result on the W&B envelope graph to see how various loads affect W&B. CFI also teaches student to *think* about weight and balance on every flight, and to calculate it precisely if there is any shadow of a doubt that it may be anywhere near the limits. Student thinks about W&B on the way to the airport, and if he's got any concerns, calculates it out.
Scenario 2: CFI teaches pilot that you MUST calculate W&B, no matter what, on every flight, to the exact pound and inch-pound. And calculate EXACT takeoff performance, EXACT climb performance, EXACT this, EXACT that. Student plays along until after the checkride, takes a few flights where he does all that stuff and the plane flies the same way it flew before. Then he goes to take a few of his buddies flying. By this point, he's tired of all of the "homework" involved in flying. The plane flew just fine every time, even when he skimped on a few calculations the last couple of flights. So, he thinks, "forget what my CFI said, he was just making me do busywork - The plane always flies just fine no matter what I do." So, he loads all his buddies into the plane, takes off overweight in a high-DA situation, stalls on departure and we have another "4 fatal" NTSB report.
You cannot teach away human nature, and humans are lazy. So, IMHO, your students will be safer if you teach them when those calculations are most important, and WHY, and make sure they at least *think* about it every flight, than they will if you just pound it into their heads with the "do this" mentality.
Not a CFI, but I did train a lot of truck drivers, and have seen the results of the "do this" variety of teaching...
Edit: Just an addition. The student *should* be able to answer correctly when you get into the plane after he preflights and ask "so, what about our weight and balance? Is it OK?" Or, replace performance for W&B, etc. If he says "Yes, I already calculated it for this plane, our weights, and the same amount of fuel last week, and the W&B numbers in the POH are the same as they were then, so we're well within the envelope" then you don't need to make him work it out again. Likewise, if you ask "So, what about density altitude?" and he answers "Well, we're using a 6,000 foot runway which is longer than anything listed in the performance charts, and I will abort the takeoff if we're not off the ground in 3,000 feet because even that will indicate that something is seriously wrong" then he has satisfied the burden of 91.103, AND showed that he understands what's going on behind the numbers and isn't just plugging things into a chart.