The precise answer is "Possibly". The other things to consider are vibration and whether there is likely to be any board flexing by attaching/detaching cables to/from the board or inserting/removing option cards (the latter probably not an issue if I've read the location of the missing standoffs correctly) .
Then the the risk of damage to the system during the procedure to fix the missing standoffs (such as scratching the PCB tracks in the case on existing standoffs
Vibration could come from a cpu with a fan, or a cpu with large heavy heatsink. Vibration could cause a fatigue failure of a soldered joint over a long period of time.
To be honest unless you think there is significant risk based on the possible issues I've hypothesised in the context of tour system and how you use it, I'd leave it alone.
There are also other possible things that could be done such as sliding a small self-adhesive rubber foot under the unsupported end of the board to add some support - if you can get it under there without having to remove the board.
Other issues with standoffs are that sometimes they are omitted as the case holes do not line up well enough, or that too many get fitted to the case, some in the wrong holes, and they short on the back of the motherboard. I've seen experienced techs get that one wrong by not paying attention (many years ago one of my roles in a past life included doing QC on outgoing custom built PCs for a system builder company).
Edited by x64, 25 May 2017 - 11:39 AM.