If i use use BERT embeddings for if cosine(sent1,sent2) > 0.9, then is it fair to assume s1 and s2 are similar
They might or might not be similar, the embeddings extracted by mean pooling the BERT output usually have high cosine similarity even though the input sentences are completely different.
Bert embeddings are not meant for sentence similarity task(SST), but there is some research combining Bert and SST.
Here are those resources,
SBERT paper: https://arxiv.org/abs/1908.10084
SBERT implementation: https://github.com/UKPLab/sentence-transformers