Mohammad,
Generally , usage of shoe depends on various factors like the line is insulated, if it has slope, heat transfer to structure etc etc. But there is no such thumbrule of using it for lines 24" & higher. You may find cases for shoes being used for lesser line sizes too.
As correctly mentioned by RS, a shoe (or saddle for larger dia pipes) provides better load distribution.
Learner,
In reply to your query, the best way to support heavier valves is to provide a support right under the valve base. However, if that is not possible then you may provide support on either side of valve as close as possibe to avoid large bending moments at flanges (which may cause leakage) & provide better maintenance!!
Edited by SJ (11/15/11 12:42 AM)
_________________________
Keep Smiling
SJ