I don’t know how used is this tag (‘is in’) but I believe it will save a lot of time to namefinders and routing applications if it is used right. Right now in Romania it is partially used, and I am not sure if we should create a rule about adding this tag (where information exists) in the database.
What do you think?
I understand what you are saying, but perhaps a better option would be to help write a good howto, and link to it here. Or help write sometemplating for Potlatch so it’s easier to enter the data you want.
It’s impossible to “create rules” about what data should go in, everyone has different goals in Openstreetmap hence they want different kind of data to be entered. If they all get what they want then you will be required to enter the plant names that are common in the area around your road.
By rules, I wanted to refer to some “good practice” document, a howto if you want. I’m absolutely sure that it is impossible to impose rules in this kind of project.
One thing that I seem to missed in the first message is that I would like to use this tag for ways, to be able to find easier, a street in a specific city. The wiki page http://wiki.openstreetmap.org/index.php/Key:is_in mentions that is recommended to use this tag ALWAYS in the case of places, it doesn’t say anything about ways. So does it make sense to tag ways that belong to a city in this way?
It seems quite a lot of work, and not particularly efficient to ‘is_in’ each element, rather than just the ‘name’ node.
If place= is applied to an area, can this data then be used to locate the ways within that area, and apply to them, or would that not really work?
Ideally some less primitive method of being ‘place specific’ to ‘ways that belong to a city’ is needed, if the concern you stated is resolved.
For the record I think is_in was a bad idea, it’s too manual. Areas would be much better, if they actually would work… 
And areas would give you more than one is_in, e.g. the same place can be in two different cities that overlap a bit.
Name finder (the search box on osm.org) will give you what city/suburb it thinks your way is in, I think this is the best option. (You will have problem in cities that are seperated by water like Torneå, Happaranda) but it works ok anyways.
is_in can not be on every way, out of practicality imo.
I agree with emj, is_in is an ugly construction for dumb software solutions. Checking against areas is much better.