In an open air case that has non metallic base, are Stand-offs for MOBO really needed?
Is there any big reason why you think the HDD and PSU's should be fixed with screws instead of having them loose but steady?
Are the standoffs nesaccary, probably not, but they cost almost nothing, why are you even asking.
Yes you should secure the powersupply and hdd with screws, again not expensive or very time consuming but makes it alot easier if you need to move them.