Which value should be used for SP for booting process?

The bootstrap sequence in the BIOS will load the first valid MBR that it finds into the computer's physical memory at address 0x7C00.

org 7c00h      ; set location counter.
mov ax, XXX    ; What is XXX?
mov sp, ax
; Now PUSH an POP are safe

Solution

Any value of SS:SP such that there's enough stack space for your code AND interrupt service routines is OK.

And, of course, your stack shouldn't collide with any of your code or other data or run into ROM or a memory-mapped device address range.

The BIOS does not guarantee what SS:SP your boot sector will receive. So, changing only SP isn't right.

You could for example do this (if there isn't any code or data of yours at this location):

...
mov ax, 0
mov ss, ax
mov sp, ax
...

This will set SS:SP to 0:0. Don't panic yet. The next push will first decrement SP from 0 to 0xFFFE and write to 0:0xFFFE, not to 0:0.

This will give you 0x10000 - (0x7c00 + 0x200) = 33280 bytes of space between the end of your boot sector and the maximum stack pointer address. That's plenty of stack space.

Also note that when changing both SS and SP, you either have to do that with interrupts disabled or first change SS and then change SP in the immediately following instruction (like shown above).

Alternatively you could use the LSS SP, ... instruction, but it takes as an argument an address of a far address, meaning your new SS:SP value would first need to be somewhere in the memory.

Yet another way to change SS and SP is to use PUSH and RETF.