There are some compatibility issues between compressor 3.5 and fcpX and compressor 4 and fcp7. You will not be able to "send" from within fcp7 to compressor4 or from within fcpX to compressor3. Which brings us to the issues of sending to compressor from within fcp.
You can select a clip or sequence or multiple clips or sequences and send to compressor (file: send: compressor). The advantages: Can save some effort and quality but ... it's never simple. Any unrendered effects will be rendered and compressed in one step possibly avoiding a quality hit and saving some time. However, you will not be able take advantage of a cluster to speed up rendering within compressor so the time savings may not exist and the whole process may actually take longer. Since effects will remain unrendered within fcp they will need to be rendered if the sequence is output again. Also, if there's a problem in your sequence or in compressor dealing with your sequence, compressor will "hang" and you will not be able to quit fcp. Note that quitting compressor does not end the compression process. You will need to force quit fcp.
For me, the ideal workflow is to export from within fcp as a selfcontained quicktime using current settings and then bring that file into compressor. This gives you a record of your sequence at that place in time that you can compress again with different settings if necessary. And if there's a problem with the output files from compressor, you can easily see if the problem lies within fcp or within compressor. The fastest workflow is probably to export a reference qt with current settings from fcp and bring this file into compressor. But remember, if any media referenced in this qt are moved or deleted, the reference qt will be unusable.