Soldering directly to an 18650 (or any battery without solder tabs) can be a bit tricky. Too long a contact with the iron and you may damage the battery. Too short and you'll get a cold solder connection that won't last. If the mod has enough room, soldering in the appropriate parts of a battery holder is a better way to go.
OT at this point but are there any good videos on replacing internal batts on regulated mods? And isn't there solderless ways to do it? :- )