DS roms (and nearly all roms for other systems for that matter) are dumped to match the size of the save chips used (8, 16, 32, 64, 128... Megabytes) and naturally the code does not have to take this all up (some like Snowboard Kids (US) are nearly half junk). The remaining bytes are filled with 00s or FF's (sometimes alternating between the two) and these can usually be safely erased (sometimes a 00 or FF is used to end a file, line, picture.... and this can mess it up).
Now with these drag and drop onbard patching carts then there is no client phase that can trim the rom and everyone seems to want to squeeze every drop of space so people are searching for trimmers to use.
As for me I still use a GBA slot cart so I am not bothered by this but:
Purely as it reads 7zip and select any and all roms I want I would use offline lists trimming function, GBATA can take care of anything else.
People that are curious check out the DS rom ripping and enhancements link in my signature as that is a damn easy way to save space as well (you can replace the movie on Castlevania and half the rom size, you can rip out the Euro languages you do not need on Nintendogs and save yourself a nice chunk of space and of course you can replace sound files to either save space or improve the game.
Out of curiousity does anyone know if there is a DS rom trimmer (the start of the 00/FF section is contained within the header)?
edit: far too slow