Hi, yes, you should plant the stems as deeply as possible, so that the
leaves are just clear of the soil. The stems will root and therefore give a
stronger root system. For the past 2 years I grew tomatoes from seeds
planted in January/February, but I've always had trouble with lanky stems
due to lack of light at this time of year (even though my seedlings were on
the window ledge). I've also had trouble putting them too early into an
unheated greenhouse, because they really get knocked back if it's a bit cold
one night. This year I've decided to wait until March to sow the seedlings.

Thanks for that. Planting them right up to the leaves would mean
planting them a good three inches deeper. Is it OK to go as deep as
that? Did your lanky stems 'beef up' sufficiently over time or did they
remain scrawny?