High Sierra reFind/ dual boot
SIP would only be a problem if you wanted to install rEFInd to your macOS boot volume. This was the preferred way to install rEFInd and its predecessor rEFIt. Currently, rEFInd is usually installed ether in a FAT32 formatted EFI partition or a ~200 MB JHFS+ (Macintosh OS Extended) formatted partition. Choosing either of these types of partitions has the following advantages.


You do not have to disable SIP to install rEFInd.
You can use the Startup Manager to set your Mac to default boot to rEFInd.


Installing to an EFI partition has the following pros and cons.


Pro: Since the partition is FAT32 formatted, you can manipulate the rEFInd files from Ubuntu.
Con: If Windows is installed and uses the EFI boot method, then you have to create a new ~200 MB partition just for rEFInd.


Installing to an JHFS+ partition has the following pros and cons.


Pro: You can add a label to the Startup Manager menu instead of the generic "EFI Boot" label.
Con: You can manipulate the rEFInd files from Ubuntu if you turn off journalling, but this still is not recommended.


I should add that you do not need rEFInd to boot Ubuntu on your Mac. Your Mac can boot Ubuntu from the Startup Manager. This has the following pros and cons.


Pro: You do not need to install rEFInd.
Con: You have to manually copy some Grub/Ubuntu files to a location compatible with the Startup Manager. If an Ubuntu update changes these files, then you would be responsible for updating the copied files.


You also can use the bless command to set your Mac to boot Ubuntu as the default. You would have to execute this command from macOS Recovery or disable SIP.

Some the questions, with an accepted answer, you might consider viewing are given below.


Triple boot macOS High Sierra, Ubuntu, and Windows without rEFInd
Dual-Booting OS X or macOS with Linux without rEFInd