Sure IAS and TAS are pretty close low down, but it's got to be a good idea to consider GS rather than these two given the track of the SID and what the wind's doing especially if you're in something with less than startling climb performance.
The extension of that is also having a hopefully cunning plan of what to do if you don't happen to be making the magic number of fpm, given the ludicrously low rate of climb on one engine that may or may not be available on the day!