Try and sit with a straight back without back support if you can. It is better for developing mindfulness and alertness.
On the other hand, if you have a physical (medical) problem that prevents you from sitting without back support then certainly use what you need.
“No lists of things to be done. The day providential to itself. The hour. There is no later. This is later. All things of grace and beauty such that one holds them to one's heart have a common provenance in pain. Their birth in grief and ashes.”
- Cormac McCarthy, The Road
Learn this from the waters:
in mountain clefts and chasms,
loud gush the streamlets,
but great rivers flow silently.
- Sutta Nipata 3.725Compassionate Hands Foundation
(Buddhist aid in Myanmar) • Buddhist Global Relief