Do 4017 decade counters sink current?
Looking at a couple of the datasheets (there are a few variants with slightly different specs), you would be much better to use a transistor as the source/sink capability is not very high.
For example the recommended operating conditions for the ST version give only 1mA source/sink typical at 5V and 25degC (whilst remaining within 0.4V of rails)
If you power it from 15V it rises to 6.8mA (3mA min), but that's still not great, considering a typical 5mm LED can be run up to 20mA (30mA in some cases)  

Here's the relevant bit of the datasheet: