I can't really answer either question with much authority but generally a buffer is most desirable at the front or the end of your chain. Buffers can help with "tone suck" but they can also color your sound (this can be a good or a bad thing depending on your ears.) Well until someone a little more knowledgeable than me comes along here is a link for a little reading. Good luck.
From General Guitar Gadgets
http://www.generalguitargadgets.com/projects/15-boostersrouters/75-transistor-buffer-projectWhat does a Buffer do?
Your signal chain is everything between your guitar and amplifier input, including cables. As your signal chain gets more complex (adding more stompboxes and patch cables) you may start to notice a loss of treble in your guitar sound. Your sound may sound much more dull than it does when you plug guitar straight into the amp with a short cable. Check out this Information about Impedance and Buffers for some information on why this happens.
This Pete Cornish Case against True Bypass Article may do something to explain why a little buffer stompbox may be something useful to you. I believe there is "A Case For True Bypass" as well, but the article might give you a good idea of what a buffer can do in your signal chain.
Please note that almost all
Boss, some Ibanez and some other commercial pedals use a non-bypassed buffer.
This means if you have a Boss pedal in your signal chain, then you probably already have a buffer in signal path even when the pedal is in bypass mode.