Say wha???
Although I agree that's it's probably safest to choose an offset deeper into the ROM, it's not necessary. There's nothing wrong with his offset, so long as he used Free Space Finder to find it. FSF (as the name implies) finds free space. If it wasn't free space and it was being used by the unmodified game, it probably wouldn't be filled with a sea of 'FF'. Also, 0x800000 is only free space for Fire Red. I believe it's used in Emerald and it's possible that it's used in Ruby/Sapphire as well.
Though I'd really appreciate it if you could clarify what you mean by "ROM space". I'm at a loss as to what exactly you were trying to say there :\
0x00000000 - 0x01FFFFFF is BIOS data within the Image of a GBA Game, the data that starts teh game and all that. (GBA games usually have the extention .gba, but sometimes .bin)
0x02000000 - 0x02FFFFFF is WRAM (This changes as you play the game, so space that might look free, will often NOT be free as you play the game.)
0x03000000 - 0x03FFFFFF is IRAM (Same as WRAM, what looks like Free Space, Wont be free when you play the game.)
0x04000000 - 0x04FFFFFF is I/O data (As before, it is NOT free space when you play the game.)
0x05000000 - 0x05FFFFFF is Pallet data (Which DOES change as you play teh game, loading different pallets and such.)
0x60000000 - 0x06FFFFFF is VRAM (Once again, NOT free, FF's and 00's WILL change when you play the game.)
And finally.
0x70000000 - 0x07FFFFFF is OAM (This data holds sprites, so if a map has TONS of different sprites, grass, and other animated objects, it will often be loaded into here, so it too is not free space.)
0x08000000 - 0x0FFFFFFF is ROM space, data that can not be edited with commands such as writebytetooffset, it holds data such as compressed images, music, and, scripts, amongst other things that the different RAM's read and load.
It is safest to start inserting data at 0x800000 (Except in Emeralds case, where a location further into the ROM is preffered.) because it wont have any possibility of randomly changing during gameplay.