The way I see it, there are two aspects of man D - playing shut-down defense and creating blocks. The ideal you should strive for is to be able to shut down your man while being able to get a block every once in a while, or be able to get blocks while not letting your man get into dangerous positions on the field. In some ways, these two aspects are contradictory to each other (i.e. if you are entirely focused on shut down defense, you may miss an opportunity to get a block, and if you poach off your man you will obviously not be shutting him down). So, it's important to be able to do each one, and once you can do each of these then the combination of knowing when to do what.
In practice, you may want to go into a defensive point with the mindset that, for the whole point, you are going to deny your man from ever touching the disc. Likewise, you might want to take the mindset that you are going to employ any means necessary to get a block. Sometimes it will work but obviously not always. (Note: If your captain/coach/teammate yells at you for not playing the way they want you to play, make sure not to tell him/her I said it was okay. It's just something that might be helpful in working on developing each aspect of the so-called defensive mindset.) I guess that the overall message here is to play shutdown D but try tobe aware of any possibilities to get a block. When that opportunity presents itself (or a split second before it does, if you can predict what's about to happen), it's sometimes o.k. to go for the block instead of staying right on your guy ( the block could come either in the open lane, deep, around the dump, or somewhere else on the field, depending on how the play develops).But, get back to your guy ASAP once the chance for the block has decreased or disappeared. Or, if you baited the thrower into making a bad throw toyour guy, lay out past him and get the D.