Culturing Cells
Ideal method for culturing cells: Start an overnight culture from a frozen stock and use that overnight culture for work. Always start from the frozen stock.
Less ideal method but still acceptable in a pinch: Start an overnight culture from a frozen stock, store that overnight culture at 4C, and use that overnight culture for further subdilutions and work. Usually, I use cultures left in the 4C for starting new cultures for about 3-5 days at most.
Bad method: Subdilute a stock to create a new working stock, and then use that new working stock to subdilute into another stock, and so on. This is a bad method and should be avoided.
Maintaining Plasmids
In addition, although wild-type E. coli (such as MG1655 or EMG2) may seem to yield higher efficiencies for transformation, you should try to avoid maintaining plasmids in wild-type strains. Instead, use suitable cloning cells, which have useful properties such as being endA deficient (leading to higher quality plasmid preps).
Tables of suitable cloning cells can be found online, such as http://www.neb.com/nebecomm/tech_reference/competent_cells/strainProperties.asp and http://openwetware.org/wiki/E._coli_genotypes