I usually do what my speed to fly indicator tells me. The hard part is
really getting the right MC setting. If you have a nice thermal
hitting 3m/sec on your averager, the real average on the complete
thermal will probably be below 2m/sec...taking into account the time
needed to center the theraml and weaker conditions at the top and
bottom of the thermal. Flying a bit to slow is usually less damaging
than flying a bit to fast. You also have to take into account that you
have a higher range flying slower...which in turn gives you a better
pick of good thermals. Regarding all these implications I set my MC to
about 1m/s when the averager shows thermals of 3m/sec. If conditions
ahead look good I go up a few points...if they look bad tend to be
more conservative and slow down to MC 0,5.

When flying the LS8 full of water a MC of 1m/s dictates speeds in a
range of 140km/h to 180km/h. I think that is more or less fast
enough;-)

If you want to do better on your averade speed then optimizing cruise
speed probably won't give you the desired effect. You lose most time
(and speed) thermaling so the trick ist to thermal as seldomly as
possible an when it's necessary only picking the best thermals
around;-)