Why are tides biggest during the equinoxes? I've read that during the equinoxes, tides are at their maximum. Shouldn't this only apply to latitudes near the equator? I mean, when the solar declination is near to 0º and we have a full or new moon, the combined effect of the Sun's and Moon's gravity should create the highest tides, but only near the equator (see in wikipedia the article Tides, which says that equinoctial tides are maximum)

Is this true for other latitudes?
Only a syzygial tide during/near an equinox is the strongest. This means, there must be either new or full moon. In general, syzygial tides are strong because three bodies (Earth, Moon, Sun) align near one line, and tidal effects of Moon and Sun on Earth become (nearly) collinear and sum to the maximal possible magnitude. The Moon’s orbit is inclined to the ecliptic by about 5° only (cos 5° ≈ 0.996), so two tides are aligned almost perfectly on a Moon’s syzygy during whichever season. The difference is that, during/near an equinox, this line also lies in the equatorial plane and rotational motion of Earth’s surface/hydrosphere/crust can direct the stuff to move along this tidal line to a maximal range possible. At least, on the equator.

As for variations in terrestrial latitudes, namely mid-latitudes vs equator, comparison between syzygial tides during solstice and equinox far from the equator is a mechanical, not astronomical problem. Tidal bulge should be stronger during solstice, but projection of velocity to the tidal line will be greater on equinox. A detailed analysis of particular body of liquid (and its shore) is required to compare.

In contrast, a quadratural (where Earth–Moon and Earth–Sun lines are perpendicular) tide during an equinox is as weak as any quadratural tide. The solar tide basically cancels a part of the lunar tide. A quadratural tide during an equinox might even be the weakest, since lunar tide outmatches the solar one, but on an equinox and Moon’s quadrature simultaneously the Moon definitely lies away of the equatorial plane.