There are a lot of theories and myths about battery life and I haven't seen enough statistically significant data to support any of those 'theories'. I have heard that it is bad to use a phone while it is plugged in but I will tell you why I think this is wrong.
From my own experience as an Android developer, my phone is ALWAYS plugged in when I am at my computer and it is almost always fully charged while I am using it. I have been an Android developer for about 2 years now. I program just about everyday, which means everyday my phone is plugged into my charger while I am programming. I have an old Droid 3 that I use for testing compatibility for Gingerbread (2.3.3). When I got the phone it needed to recharge every other day. Now, unless I use it constantly, it will survive for just over a day. For a 2 year old battery, that is pretty standard degeneration.
If you think about it, everyone pretty much plugs their phone in at night and it will take about an hour to two hours to charge (if the batter is completely dead). That means that the device is sitting on the charger for about 6 hours fully charged every night (if you sleep 8 hours a night). I don't think that a few hours of it on the charger while you are at work will affect your battery's lifespan enough for you to notice.
If anyone has any logical reason or statistically significant data to support the fact that this is a bad idea, then I will eat my words but from my experience I would say that your battery will not experience any significant lifespan decay from you plugging it in at work.
If you want to know more LifeHacker thinks it's a bad idea, but they give no statistical support for their tables
Over at Android Forum it seems the general consensus is that it doesn't matter.
Here at android.stackexchage it seems like the general consensus is that constant charging doesn't really hurt your battery, but you should do maintenance discharges to keep it healthy:
Although constant charging cannot hurt, it looks like you may want to periodically run the battery all the way down so the digital circuits can correctly calibrate
If you really want to get to the bottom of it you can read this article. Although, Jeff Atwood has done a pretty good summery of the article:
Avoid frequent full discharges because this puts additional strain on the battery. Several partial discharges with frequent recharges are better for lithium-ion than one deep one. Recharging a partially charged lithium-ion does not cause harm because there is no memory. (In this respect, lithium-ion differs from nickel-based batteries.) Short battery life in a laptop is mainly cause by heat rather than charge / discharge patterns.
Batteries with fuel gauge (laptops) should be calibrated by applying a deliberate full discharge once every 30 charges. Running the pack down in the equipment does this. If ignored, the fuel gauge will become increasingly less accurate and in some cases cut off the device prematurely.
Keep the lithium-ion battery cool. Avoid a hot car. For prolonged storage, keep the battery at a 40% charge level.
Consider removing the battery from a laptop when running on fixed power. (Some laptop manufacturers are concerned about dust and moisture accumulating inside the battery casing.)
Avoid purchasing spare lithium-ion batteries for later use. Observe manufacturing dates. Do not buy old stock, even if sold at clearance prices.
If you have a spare lithium-ion battery, use one to the fullest and keep the other cool by placing it in the refrigerator. Do not freeze the battery. For best results, store the battery at 40% state-of-charge.