The playerbase typically does research to find out which stats are the most important for a given class, usually referred to as "stat weights". For the most part yes, you want to stack as much of it as possible. Though, in cases like acc and skill/spellspeed, you may have a certain threshold you want to reach, and then avoid adding any more after that point.

Also, when it comes to main stats like int it's important to take into account stat caps. Every piece of armor has a maximum for each stat based on it's ilvl and slot. For HQ crafted gear, dungeon drops, and tomestone gear, most of the stats on there are at or near their cap, which means you won't be able to meld more intelligence. This also means you may need to meld the second-most important substat, such as melding crit onto BLM gear that is already capped on spellspeed.