It depends upon one's objective. State maps and street signage in Hawaii standardly use the ‘okina as do city, state and national building placards. Lonely Planet guides do, a NatGeo map of Hawaii poster does not.
The ‘okina is a pronounciation guide. I am from the US MidWest. Being familiar with U-Haul misled me into saying "yookoolaylay" when I arrived here three decades ago! It is also an identifier between otherwise identically spelled words; ‘ulu is "breadfruit" while ulu means "to grow".
If I were texting with a Spanish friend and I wanted to say "mañana" in an, until then, exclusively English conversation I would use the "ñ". I would use the same guidelines for Hawaiian words / friends.