I've just bought a Wilton #233 nozzle for piping grass and hair etc. shall i use royal icing or buttercream? i've always piped with royal icing before but everyone seems to use buttercream with this grass effect tip; is there something i'm missing?!

Nope. Use either one. Whatever works best for you.

Doesn't it just depend on what you want to end up with? If you want something people can cut into and eat that tastes like icing, go with buttercream. If it's ok for it to be hard, then royal. I think both will pipe the same, but the royal may give you sharper edges.

Thanks! if i've got any buttercream left over then i'll colour and use that. if not i'll make up a bit of royal icing

The one time I used the tip to make grass I used buttercream. You can see it on my jungle cake. It turned out great. It crusted a little so it held up well but you could still eat it without it being firm like royal icing.

There is no hard rule for the grass tip. If you are doing permanent display I would use RI if you are eating it then BC, but if you are doing cookies then RI. HTH