Recently being in Cardiff, I’ve just plunged into the shop or caffee saying Coffi Americaneur plis (of course I’ve said my greeting in Welsh first) not even asking if they speak Welsh or not. Getting some puzzled looks many times I’ve usually continued: “Oh, sorry, you don’t speak Welsh? OK, then we probably should continue in English. One American coffee please.” and I went on … “Mawr plis … gyda llaeth … oh, with milk please …” and I usually just gesturred that “mawr” word for them. Then it turned out or they really didn’t speak Welsh at all, or they did but were so puzzled to hear it because they talked to people before me in English so they needed some seconds to adapt to my “agression” on them. Then only I usually told them I’m learning Welsh and I’m actually even not Welsh at all but come from Slovenia. From puzzled surprised people they usually turned into surprised pleasant ones. So, despite it maybe was a bit rude from me to make “attack” it worked just fine for both sides.
So, this for, I believe you should never say your Welsh is bad but rather tell people you’re learning the language so they should maybe be a bit patient with you! You will soon see it works that way too. I was taught on this forum particularly that one should not neglect themselves so I FINALLY obayed that rule and went for speaking whatever quality rather then excuse myself at the very beginning for not speaking fluently/good (or whatever). If you’ll not impress people with your Welsh then you will do so with your courage to just plunge into conversation no matter how short or at the “beginners” level it might be.
Hwyl! 
Tatjana