While the lowpass (LP) before analog-to-digital conversion (ADC) is generally required in order to avoid aliasing effects the reconstruction lowpass after digital-to-analog conversion (DAC) can be omitted if the bandwidth of the analog signal is allowed to have a high bandwidth.
In radio communication systems the reconstruction LP is very important because neighbouring channels must not be distorted and bandwidth is a valuable ressource. Thus the transmit signal (1) bandwidth should be as low as possible.
But especially if you transmit your signal over a lowpass channel (e.g. wireline) and you have no frequency-multiplexed signals the reconstruction LP might be avoidable. I would consider the following points for the decision
- What's the bandwidth of the DAC output signal?
- Is there an amplifier after the DAC? Can it handle high frequencies?
- Is a modulator used? Can it handle high frequencies?
- Are there any other signals on the transmission medium and can they get distorted by your transmit signal?
- Are there non-linear components that could cause mixing of higher frequencies into the baseband (i.e. into the usefule frequency band)?
As it has been noted before, some components of your transmitter (DAC, Amp, ...) or the channel might already reduce the bandwidth of your signal considerably so that an additional LP is not required.
(1) With transmit signal I mean the analogue signal.