It's safe to treat malaria even when you haven't been tested. As you said, majority of people in Nigeria have the malaria parasite in their blood almost all the time whether symptomatic or not.
When anyone who lives in malaria endemic region presents with any sickness, it's safe to treat malaria alongside whatever it is that is wrong with the person because other sickness or even stress reduces ones immunity which allow the already present malaria to further proliferate and cause more damage.
Most often than not, getting a test result in our environment could be a challenge, then we can say it's safe to commence antimalarial to avoid the individual coming down with malaria complications like;

1. Cerebral malaria which can result in coma.
2. Febrile convulsion
3. Acute Kidney injury
4. Hypoglycemic coma
5. Anaemia and anaemic heart Failure
6. Jaundice or Hyperbilirubinemia
7. Hyperparasitaemia
8. Prostration (complete loss of strength)
   The list goes on and on. Complications are more common among children especially under-5 and pregnant women, however they can occur in a normal adult mostly when there is a delay in treatment.

It's therefore safe to commence antimalarial early when you have the symptoms. As a matter of fact any foreigner coming to Nigeria should take prophylaxis against malaria because they lack the partial immunity against malaria just like in children, risk of complication is high.

I believe this answers your question.
Thank you.