Its not really relevant. The ppi is to determine the physical size, whether you multiply by inches or you divide by pixels.
When it comes to the web, monitors, tv's, projector's and any other display, the main factor is always the pixel dimension, which is also how a monitor is listed in their specs.
BTW when you export using save for web, it strips the ppi value from the metadata. This allows another app to decide what that ppi will be which is usually a default value, like photoshop for example would be 72ppi.