They can ignore the body of Christ in Heaven if they want, and in the same way they don’t have to ask anyone to pray for them either, although they should. The Bible is filled with people praying for others, and James tells us the prayers of a righteous man availeth much. (James 5:16)
Hebrews 12:1 shows us a cloud of witnesses in Heaven essentially rooting for our victory; it would be worth it to employ their prayers, since they are righteous.
Likewise, the Blessed Virgin Mary has appeared in numerous apparitions encouraging the Rosary, which includes the Hail Mary, an obvious endorsement of her intercession.
So, no one has to [(pray to|ask the help of) the saints or angels], but they would be foolish not to.