What E-value BLAST cut-off is best?
There is no perfect cut-off. It always depends on what you're doing. The e-value is basically a measure of how many such alignments you would expect to find in a database this size by chance. Therefore, e-values greater than 1 mean that you'd expect at least one alignment similar to what you've found by chance alone. 

As others have stated, the e-value is not a rigorous measure of significance. It depends on


The length of your alignment.
The alignment's bit-score score (this takes into account amino acid substitution scores based on the matrix being used, BLOSUM65 by default).
The size of the database you are searching in.


I have at different times and for different projects used e-values ranging from 10 to 1e-100 or less. You always need to take into account what you are looking for. For example, to run a tBLASTn looking for distant homologs, you'd use a relatively high e-value, say 1e-10 or even 1 or greater depending on the details. For example, the default e-value for ncbi-tBLASTn is 10.

To find extremely similar sequences in closely related species, you'd use a much smaller one. Say 1e-20 or 1e-50. Normally, you tweak these settings after seeing your results. 

After running a few hundred blasts, you get the hang of it and you can have a pretty good estimate of the e-value cutoff you want to use. Until you get that experience, you will have to use trial and error. Just remember that the conclusions you can draw from your data will be heavily influenced by that e-value. Don't try to publish something claiming you've found a homolog if the supporting BLAST hit has an e-value of 45 for example. 

Personally, when looking at BLAST out files, I check the e-value, the alignment's length (as a percentage of the query), the alignment's %id score and the bit score itself. Of these, the most reliable is the bit-score but, again, that too changes depending on what question you are attempting to answer.