To say something about this problem and not just moralize.
Led's are usually driven in current mode, as they have low dynamic resistance when open. You can use aproximation circuit which is made from voltage source and resistor. I draw a picture, the graph on left is from datasheet of standard super bright red LED, I added an aprox. line. As you can see, slight change in voltage is gonna make much bigger change in current. To drive it by voltage is not therefore wise, however you can still do it in some cases (f.e. from battery that you know will not exceed certain voltage). Anyway, current is ALWAYS directly deppending on voltage, you can't have 20mA running through LED at 12V, ever...
There are several types of LED-strips on market. You can get current driven strips, that has no resistors and are a bit more efficient, but you need special current source for it. Standard 1210 strips has 3 diodes per segment and build in resistor (usually about 3V per diode and 3V on resistor). This resistor is all you need to drive them by voltage. F. e. in this case, 13V at input will give you current about 26mA, which is still pretty safe (with just a bit lower life expectancy). Lenght of segment is usually 5 or 2,5cm, 20mA per segment, so about 400mA or 800mA per meter.
Strips with 5050 diodes are a bit different as those diodes are in fact tripple LEDs (they have literarry three pn junctions in one device). Making the current about 1,2A or 2,4A per meter.