Since I do not use my PC most of the time during the day, just when I need to research, browse the net, opening my emails, then is a scheduled daily memory scan really neccessary in that case?
I do use my PC, but it is not on all day. And I keep it turned off when I do not use it.
Or, should I just shcedule a daily scan anyways just to be on the safe side?
Also, should I check RUN IF MISSED checkbox as well or should I leave that blank?
You should perform this scan for ALL machines once per day under normal circumstances. There is never a time when you don't do this. (During Outbreaks we increase this to once per hour if we're trying to erradicate a pesky virus.)
I would recommend that you run this with settings like "Every day at Noon -and- Run Missed Task" that way you always get it. Also, if you are only scanning memory then this scan takes about 30 seconds on a PC built in the last 3-4 years. Since it isn't touching any disk, it will have no discernable impact.