Sisiutil
All Leader Challenger
If you settle on the resource, you automatically have access to it. You just lose some of its output (food, hammers, and/or commerce).Duly noted.
Though it's tough to teach an old Darth new tricks. I'd much rather actually have the resource itself, particularly if it's of military value (Like Iron).
F
Generally, you want to avoid, except in extremely unusual circumstances, settling on top of a food resource, as this will cost you some food, and food is king in this game--crucial to growth and, by corollary, to the ultra-powerful Slavery civic.
After that, you want to try to avoid settling on high commerce resources (gold, gems, silver, dye), especially early in the game when working one or more of those resources can really boost your research. The exception to this rule is incense, since it's usually on low-food desert tiles.
Settling on high-hammer resources (especially the crucial early-games ones, copper and iron) will deprive you of some hammers, but brings the resource on-line instantly (no mine, quarry, or road needed) and makes it darn-near pillage-proof. Base your decision on your plans for the city and the map situation.
Other resources (such as wine, silk, ivory, furs, etc.) are less spectacular with their yields. If settling on top of one of these will provide you with a better city, go for it.