Enabling and disabling run time configuration parameters.

I have discovered that I could optimize queries by adjusting the 
following parameters such as enable_seqscan, enable_hashjoin, 
enable_mergejoin and enable_nestloop.

Is it a good idea, to temporarily adjust those values before running a 
query to spend up the execution time?  I've searched online and wasn't 
able to find articles about it.

I need to speed up an enterprise application that I'm working on, and I 
wouldn't want to screw things up.

My plan is for every query that could be optimized by adjusting 
parameters: I'll enable parameters that'll speed it up, run the query, 
then set the parameters back to their default values.

Thanks in advance.