is it better to walk from jiri to lukla
or from lukla to jiri ???
thinking that the walk in will good for acclimatisation but will it tire me out for the rest of the trek ??
if i fly out, do i need to book it before i leave kathmandu ?? just that i will not know my exact return date !!


I would say walk in/fly out is better option. Though one thinks it is easier to walk out (going downhill) in fact there is so much ascents/descends between Jiri and Lukla it really does not matter if you are going up or down. You should know that Jiri to Lukla section is more strenous than Khumbu so trek after Namche is more like holiday after a hard work. On the other hand I dont think it will tire you, more likely you will become more fit and will cope with Khumbu part with ease.
You dont get much acclimatized on the way to Lukla since you are staying below Namche altitude all the way so acclimatization (and rest) in Namche to go further on is essential.

walk in fly out,,,,,,,,,,,,,,the walk from jiri to lukla is ammmmmmmmmmmmmmmmmmmmazing
Walk in (or in AND out)! You'll only be fit, not tired, once you're closer to the BIG ones. Oh, and do all three valleys: Gokyo, Lobuche and Chukhung! If you don't have the time for all three, you're better off flying in and out. I did the whole thing walking in 28 days a year ago.

You need the acclimatisation, so you better walk in. The moment you return from the trip and start on that first beer in Lukla, there is nooooooooo way you walk out hahahahaha.
If you go in March-April-May, you better book the whole thing BEFORE you go, because there will be plenty of fellow travellers hiking/trekking aroud. There is an office in Lukla and Namche Bazar if you want to change, I've never had problems changing a flight.
One more : never mind the airline company : they have the same planes, the same everything. Just take the cheapest ticket !
Good luck,
Peter