Whether you'll lose an appreciable amount of signal due to reflections from an unterminated transmitter port, I'm not sure.
Well, lets say you have a 4-way passive splitter. You connect an RF generator to the input, and on one output you connect an RF signal meter.If you look at the (simplified) block diagram of the Sennheiser ASA1 in the owner's manual (https://en-us.sennheiser.com/global-downloads/file/7101/ASA1_Manual_06_2016_EN.pdf) it appears to show an RF amplifier followed by a passive splitter block, so for a passive splitter block to work optimally, all the inputs and outputs need to 'see' the correct impedance (Load). (The Sennheiser ASP1, however, is a completely passive splitter with only a DC injection circuit to power the remote antennas.) It generally can't hurt to terminate things to the recommended impedance, so when in doubt, do the right thing and you will probably be OK. I find it curious that Sennheiser doesn't bother to explicitly say anything one way or another - it seems like it would reduce the calls to their support department. At the very least, it will keep the local RF guru from seeing an unterminated line and having a coronary attack.