Or does 25mm tend to be a true 25mm?
Tubulars tend, on the whole, to be pretty true to width. Depending upon construction their height, however, can differ a bit. Recall the shape of the tyre, in contrast to clinchers, is immutable to rim design. Tubular tyres are round. Given variation in roundness, etc. I'd avoid overly tight fits. Most frames, however, will accommodate 25mm tubulars. Some bicycles might have problems fitting Clement Campionato del Mondo (28mm) but they've been out of production for more than 30 years..