The documentation says I need a null modem cable. My Routerboard 532’s are all up on towers. Can I do a net install withouth the null modem cable?
you do not need a null modem cable to do netinstall. where did you read this? you only need an ethernet connection to the router
you need to set RB to boot form ethernet, then hook up that to PC with running netinstall and everything else you have to do will happen on your netinstall window.you have to configure netinstall accordingly 
good luck
From the Netinstall page:
NetInstall
Description
NetInstall is a program that allows you to install MikroTiK RouterOS on a dedicated PC or RouterBoard via Ethernet network. All you need is a blank floppy or an Ethernet device that supports PXE (like RouterBoard 100, RouterBoard 200 and RouterBoard 500 series), an Ethernet network between workstation and dedicated computer, and a serial null-modem console cable (for RouterBoard routers).
Nowhere does it say it is “optional.” Glad to know it is.
you have to change the boot sequence. this can be done from the system routerboard settings menu I beleive, but it’s probably fastest to do with a serial cable, of course this is only when the routerboard is sitting in front of you…
I don’t think this was selectable from the commandline until sometime after 2.9.5, but I’m not sure on that…