I understand your question as "Sometimes 'th' is hard for me. Is it OK if I don't say it properly?" Of course if you want to speak English well, the answer is no, it's not OK. Some students try shortcuts and use a substitute, and it always sounds unnatural. I too had to learn and re-learn sounds to speak a different language, and my personal experience tells me that avoiding the problem leads to misunderstandings, comedy and the wrong train station. Admittedly, words like "sixth" and "truths" need a little concentration. Practice moving your tongue between the 's' and the 'th', and it will become easier in time. Perhaps you could sit down with a native speaker and ask him or her to show you exactly how it's done. You can also ask the speaker if you are pronouncing 'th' properly. Sometimes the problem exists because the sound you want to make isn't right in the first place.

Yes, 'th' is pronounced as either /ð/ or /θ/ The 'th' is pronounced at the end of the word 'sixth', but it's very slight. It's not nearly as long or as clearly pronounced as a 'th' at the beginning or end of a word. As you are finishing the /ks/ sound to say the 'x' you just lightly put your tongue behind your top teeth for a second. That's all you need to do to end the word, and to make it different from 'six'. Try it!

'th' is almost always pronounced as /ð/ or /θ/. The River Thames (/tɛmz/) is the only exception I can think of at the minute. It's not important for you to pronounce 'th' like a native speaker. Individual phoneme pronunciation is much less important than sentence stress and intonation. Everyone will understand you if you pronounce /θ/ as /s/ and /θ/ as /z/.