When is strict sterile technique necessary? Cloning vs. protein expression
For most plasmid preparation, strict sterile technique is not necessary. The antibiotics in the media will select for bacteria which carry a resistant plasmid, so you don't usually have to worry about other organisms getting in.

However, when you're using several types of plasmid, you do need to be careful to avoid cross-contamination, because each plasmid is resistant, so bacteria transformed with plasmid A will live just fine on plasmid B's plate. When plating your cells, be sure to pass your cell spreader through a flame. It doesn't have to be an official Bunsen burner, I used an alcohol burner just fine. 

If you make a big bottle of media and use small aliquots at a time, then the bottle should only be opened inside the hood.