Technically speaking, peak level charging for extended periods can stress the LiPo cells and lead to reduced life of the battery. When you leave a fully charged phone on a charger for extended periods, it goes through a ton of little discharge/charge cycles, and LiPo cells really don't like that all that much when they are fully charged. When you look at how pros charge LiPo cells.... we're talking guys who's job (and maybe life) relies on having that device work.... those batteries are often charged on special chargers that keep the battery at an artificially low voltage (i.e. charge) level to avoid cell stress... maybe 90-95%. So they can sit there all day and night for who knows how long without going to pot.
If you want that LiPo battery to last as long as it can, charging to its highest possible level for the longest amount of time, you only need to do three simple things (in order of importance):
1) Avoid extreme heat... don't leave the sucker on your dash in the summer
2) Avoid constantly going through deep recharge cycles (100% to 5% back to 100%, etc)
3) When it is fully charged, take it off the charger
Now, having it sit overnight now and then, that probably won't be much of a problem. But I wouldn't make a habit of it.