Host a tasting and someone will tell you the glass matters. They're right, but not in the way the glassware industry would like. Shape changes how much you smell, and smell is most of taste. Everything past that is decoration.
What the shape is doing
A glass that narrows at the top collects the aromatics coming off the spirit and funnels them to your nose. A straight-sided tumbler lets them escape. That's the entire mechanism, and it's why a Glencairn or a copita improves whisky noticeably while a rocks glass doesn't.
It also means the effect scales with proof. On a 40% pour it's a real but modest difference. On a cask-strength whisky or an overproof rum, a narrow glass concentrates the ethanol along with the aromatics, and you'll want to add a few drops of water to get past the burn.
The honest minimum
One narrow glass per guest. Glencairns run about six dollars each and are the single upgrade worth buying. If you're not buying anything, a small wine glass is the closest thing in most kitchens and works nearly as well.
Water glasses. Separate from the tasting glass, room temperature, and more of them than you think.
Something to spit or dump into. One cup per guest, or a shared pitcher. People skip this and then either drink everything or feel rude pouring it out. Six pours at half an ounce is three ounces of spirit per person, which is two stiff drinks.
What you can skip
Decanters do nothing for spirits. Whisky stones chill without diluting, which is backwards, because dilution is what opens a whisky up. Per-spirit glass shapes are mostly marketing; the same narrow glass handles whisky, tequila, mezcal, and rum neat.
The exception is tiki, where the mug is the point. A Mai Tai served in a Glencairn is technically correct and no fun at all.
Labeling beats glassware
If you're running six pours, numbered labels matter more than the glass. A strip of masking tape and a marker keeps a blind flight honest and costs nothing. Our guide to setting up a tasting station covers the full table layout, and every kit ships with a printable scorecard to write those numbers on.