Is charging 18650 lithium ion cells with lower current better?
Instead of thinking about what charging regimes will prolong battery life, it's probably best to flip that on its head and say what use regimes will reduce the number of cycles, and then avoid those.

The best way to kill Lithiums is to charge to too high voltage, or discharge to too low voltage. Sacrifice some capacity by charging to less than 4.2v, and stopping before you get to the end point, and you'll avoid killing your cells. Pro-tip, this is what electric car manufacturers do to be able to give an 8 year warranty on the traction battery pack.

The next way to reduce battery life is to run them too hot.

Once you've taken car of voltage limits and temperature, high charge and discharge currents can reduce the life, but not by as much as those first two factors. A low current does not reduce life. 

The only way a low charging current might contribute to a reduced life is in the hands of an inexperienced designer who thinks that lithium cells behave like nickel or lead, and that if the current is low enough, then a gentle overcharge is permissible. With lithiums, no overcharge is ever permissible, see the second paragraph.