Cutting item prices across the board is okay, if you also plan to cut the treasure tables and wealth guidelines while you're at it. But that's more about rebalancing the economy than about item creation issues. It belongs in another discussion
Would re-pricing magical ammunition equate to power creep? Perhaps. Right now, magical ammunition exists as a DM's convenience for lower level parties. A DM can hand out 10 +1 magic arrows as a relatively low end treasure, before the game gets up to the level where people start getting permanent magic items. That's good until what, level 2? 3?
After that a +1 magical bow isn't unreasonable, and the magic arrows get to sit around in someone's quiver, unused.
Was the increase of prices for some items during the 3.0/3.5 changeover a power reduction? Yes, in a way. A lot of PCs got converted and kept their now-expensive items. It effectively handed wealth to them, but it was wealth they woud probably never spend.
Now, is power creep a bad thing? Is it limited to items?
The first question is a philosophical one, without a clear answer. I don't like it, but the popularity of Pathfinder shows that there a lot of people who disagree.
The second question is a clear and resounding "No!". Later books started adding spells that gave more power at lower levels. Lesser Vigor, for example, is a 1st level healing spell that acts like a maximized Cure Light Wounds. While it doesn't grant the instant gratification of Cure Light, in that the healing takes place over a period of several rounds, it's clearly superior in pretty much every other way. You can even cast it on someone as they enter battle, so they fast heal as the damage comes in.
Similarly, the later books give new and better damage spells, protection spells, transport spells and utility spells, pretty much across the board, and all are at lower levels than any comparable counterpart.
Now, the only reason I bring that up, it being off topic, is that it impacts magic item creation. Wands, potions, staves and rings of these new spells are inherently more powerful, for the same spell level, than items from the DMG, yet they're still priced by the DMG formula even if you never even glance at the MIC.