No it is not compulsory to do an extension. Border hop with METV at Mae Sai will result in another 60-day entry. Compare w/extension 30 days 1900 baht vs. Mae Sai run from CM by bus (320-638 baht, depending on class) + 500b for the Myanmar border run). So border run is a bit cheaper, but takes a whole day, +60 vs. 30 days.
******************************************************************