I`ve read about letting houseplants 'rest' during their 'off season'BUT I`ve
also heard of people giving their plants fertilizers all year long Which is
best?

Like many other questions, the answer is: it depends.
If you have a tropical, forest floor foliage plant, like Aglaonema, which grows
slowly all year round in low light, it can benefit from occasional fertilizer
all winter. If you have a plant whose natural growth habit is seasonal, like
Christmas cactus, you water & feed it heavily all summer, & keep it drier,
cooler, & unfed in the fall until it blooms. Use common sense & read up on the
individual plant.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"The trouble with people is not that they don't know but that they know so much
that ain't so."
Josh Billings (Henry Wheeler Shaw), 1818-1885