In my experience, tickling is standard operating procedure in the Show-Him/Her-I-Like-Them-Without-Having-to-Come-Out-and-Say-It playbook. Unless you've been friends for a very long time, or have already been through/discussed mutual romantic interest, I'd say tickling is a pretty clear indicator of attraction.
There are, of course, exceptions: girls who are, by nature, particularly physical or touchy may see tickling as harmless, and do it without any sort of ulterior motivation. As guys we have to be wary of this, as we may interpret it as some sort of signal and then end up in a whole mess of trouble. There are also guys who are the same way: naturally touchy people. This is much rarer for men though, so I'd say that any guy who tickles is either romantically interested or a pig who is looking for an excuse to touch women.
As general advice, I say this: no matter your gender, I'd say don't tickle someone unless you're romantically attracted to them.