Because the math for monster scaling is so simple and transparent in 4e all of the basic numbers (defenses, HP, attack bonuses, and damage) will add up with no problems. That's part of why customizing monsters is so easy and painless in this edition (cuts down on prep time, too!). So what's the problem perceived by the DMG authors? Most likely, it's status effects and riders that are tacked onto powers, or other traits such as auras, conditional damage, and off-action attacks (or number of attacks). Not surprisingly, monsters get more dangerous at higher levels than their raw base stats can account for, which makes sense because players get more access to feats, items, powers, and paragon path/epic destiny features. Leveling an Epic monster down for Heroic PCs would make for a disproportionately challenging encounter (possibly impossible in some cases), and leveling a Heroic monster up to Epic might make for a cakewalk.
For example, lets look at the Efreet Flamestrider, a level 23 Skirmisher. I picked this guy at "random" since he was the highest level standard monster in the MV. He has an aura that multiplies ongoing fire damage, a MBA that inflicts ongoing fire damage on a hit, and an at-will ranged attack that both immobilizes and deals ongoing fire damage (save ends both). And of course there's the at-will 20 square teleport (even if it does require fire, which any DM will build into the encounter while using this creature). Even leveled down, Heroic characters don't have enough ways to trigger saving throws to easily counter his ongoing damage and immobilization (and teleports will be much more rare as well). Their healing ability is also more limited, so PCs within the aura can easily find themselves overwhelmed by raw damage. Finally, Heroic PCs will be more dependent on status effects like immobilized, slowed and restrained because they probably won't yet have wide access to dominates, stuns, and blinds, meaning that the efreet's teleport makes him very tough to hinder. Likewise, a level 1 Goblin Cutthroat with just straight damage and some minor shifting tricks won't be much of a problem for Epic tier PCs.
After pointing out the problems of over-leveling, I'm still not going to advise against doing it, at least assuming you're reasonably familiar with the system. The thing is, after pointing out the problems with the efreet simple common sense (and possibly a quick comparison with the Goblin Cutthroat) can inform which further changes must be made. It's clearly too powerful when leveled down without further adjustments, but what if we made the following changes on the fly:
- The ranged attack becomes a recharge or encounter power so it can't constantly immobilize PCs.
- The teleport becomes an encounter power so it can't constantly escape the common status effects and re-position itself.
- Either get rid of the aura, or get rid of the ongoing damage with the MBA. Specifically, either have ongoing 5 damage and no aura, or have no inherent ongoing damage with the attack but have the aura inflict ongoing 5 damage if a creature is hit with a fire attack. These 2 methods look identical, but keep in mind that with the latter option a ranged fire attack from a second monster can gain ongoing damage against PCs in an efreet's aura.
So now that I've demonstrated that you can level monsters across the entire spectrum of levels for just a little bit more work than with a more narrow level range, why should you? It's mostly a matter of personal taste. Do you want a specific monster for your adventure? Use it regardless of its base level! What if an off-level monster has a set of abilities that you really want to use for something else? Level it down/up and re-flavor it, or tack its abilities onto an appropriate-level monster. Are your PCs sick of fighting the same orcs over and over again? Find a different monster with some interesting mechanics, level it as appropriate, and call it an orc! Or, simply modify the existing orcs by giving them different abilities. One of the great strengths of 4e is its modularity. It's not only possible to mix and match elements from different monsters and different levels, but it's fairly easy as well! So when you plan encounters, don't necessarily limit yourself to the pool of monsters in a given level range; branch out for more ideas and give the players something unexpected (especially important if your players also DM, and thus have access to the same monsters that you do).