No it isn't, this appear rude because ~Quiero~ is the imperative(we use imperative for give orders like ''RUN!'' that in spanish is ''Corre") form of the verb ~Querer~. So if you want to be more polite you will say ~Quisiera~ instead of ~Querer~. So you will ordening like this "Quisiera.......(the food or thing that you want), por favor" For sum up, use ''Quisiera'' instead of ''Quiero'' to appear more polite.

Do you mind a tea with sugar? in spain, a waiter is always going to bring you a tea with sugar, so, Quisiera un te, por favor (it would be enough) Un saludo

Burt, in Spanish don't forget never "porfavor" when you ask something and "gracias" when they give it to you. Even if you pay for that. Other way besides Elba's one could be: "Deseo un taza de té" please.

Thanks Ilba. That does sound much more polite.the other way was much to rude.

I would say: Me gustaría te dulce, por favor. Eventhough you say "quiero te dulce" ....if you add the magic phrase "por favor", that is very nice and polite. So never forget to add "por favor" :)