What is more correct to use to centralize this case

If you add expand=True to the call to pack, it will be centered. expand tells the packer to expand the allocated space to consume all extra space. By default the frame will be centered in the space allocated. The net result is that the frame will be centered in the window.

frm_login.pack(side="left", expand=True)

I think this will answer your question. When you are using tkinter .grid to place your elements, you can define the weight of each column and each row. An example is the following: Let's say that I want my column number 0 to occupy less than my column number 1, for this:

main.grid_columnconfigure(0, weight=1)
main.grid_columnconfigure(1, weight=3)

In this example, the column number 1 triples the value of column 0.

This way, the elements that are placed in a column will adjust to the weight of that same column. You can do this with each column and with each row. Greetings and good luck!

Use the tkinter grid method and pass your parameter sticky="nsew". Use rowconfigure() and columnconfigure(). Set the parameter weight=1 in rowconfigure and columnconfigure. So your widget is going to fill in the whole space they can using sticky. When you configure row and column, only the rows and columns you specified are going to be in the frame with equal weight if all are configured the same. This code should work:

Note: You can use padx and pady if the frame is expanding too much. Use them when you are calling grid() but entirely depends on your purpose.

import tkinter as tk
from tkinter import ttk

principal = tk.Tk()
principal.title("Login")
principal.resizable(False, False)
largura = 300
altura = 200
posx = int(principal.winfo_screenwidth() / 2 - largura / 2)
posy = int(principal.winfo_screenheight() / 2 - altura / 1.2)
principal.geometry("{0}x{1}+{2}+{3}".format(largura, altura, posx, posy))

principal.rowconfigure(0, weight=1)
principal.columnconfigure(0, weight=1)

frm_login = ttk.Frame(principal)
frm_login.grid(row=0, column=0, sticky="nsew")

frm_login.rowconfigure((0, 1, 2), weight=1)
frm_login.columnconfigure((0, 1, 2), weight=1)

lb_usuario = ttk.Label(frm_login, text="Usuário")
lb_usuario.grid(row=0, column=0, padx=5, pady=5, sticky="e")
ed_usuario = ttk.Entry(frm_login, width=24)
ed_usuario.grid(row=0, column=1, sticky="w")

lb_senha = ttk.Label(frm_login, text="Senha")
lb_senha.grid(row=1, column=0, padx=5, pady=5, sticky="e")
ed_senha = ttk.Entry(frm_login, width=24)
ed_senha.grid(row=1, column=1, sticky="w")

frm_botoes = ttk.Frame(frm_login)
frm_botoes.grid(row=2, column=1, pady=5, sticky="w")
bt_entrar = ttk.Button(frm_botoes, text="Entrar")
bt_entrar.grid(row=1, column=1)
bt_sair = ttk.Button(frm_botoes, text="Sair")
bt_sair.grid(row=1, column=2)

principal.mainloop()

Hopefully this answers your question.