The last time I booked on MH, I declined to pay to reserve a seat when booking on their website, assuming I could select for free afterwards. This didn't happen. On MMB, I couldn't even pay for seats. I called the London office, and was told I couldn't select a seat now. At OLCI I was preassigned a seat I couldn't then change
However, I believe YMMV.
(to be fair to MH, I only paid £60 rtn KUL-RGN, so got an amazing deal, which they no doubt lost money on, given all my lounging in KUL!)