If the storage is going to be down for an extended period of time, I may be worried about an APD(all paths down) condition which could cause your hosts to disconnect from vcenter eventually. If this is a quick storage reboot, I am doubting that the APD would affect you.
That said, you could mask all the Luns before you power off the storage, and then unmask them afterwards to ensure the Esx hosts will definitely not have any issues.
Here is a KB on it, and it can be kind of tricky to be honest:
VMware KB: Masking a LUN from ESX and ESXi using the MASK_PATH plug-in