Is it common for a character to sing in OP/ED? Some anime used their seiyuu (voice actor/actress) for their opening or ending song, I know there's some anime that used their seiyuu to sing OP/ED like The World God Only Knows, Monogatari Series, or Mahou Shoujo Madoka Magica.

Is it common for them to sing OP/ED?
The simplest way to answer this question is to take a look at a sample of anime and see how many of them have OPs/EDs sung by the show's voice actors. 

For the sake of argument, I will look at all shows with at least 10 episodes that began airing in winter 2014, excluding children's shows. I will only examine those OPs and EDs that were aired at some point between January and March 2014.


Buddy Complex - neither
Chuunibyou Demo Koi ga Shitai! Ren - ED by VAs
D-Frag - ED by VAs
Go! Go! 575 - OP by VAs; no ED
Hamatora - neither
Hoozuki no Reitetsu - OP and ED3 ("Caramel Peach Jam") by VAs; ED2 ("Parallax View") features a VA
ImoCho - OP and ED by VAs
Inari, Konkon, Koi Iroha. - episode 9 ED by a VA
Mahou Sensou - neither
Maken-ki! Two - OP by idol group that includes some of the VAs
Mikakunin de Shinkoukei - OP and ED by VAs
Minna Atsumare! Falcom Gakuen - [note: no ED]
Nisekoi - ED1 ("Heart Pattern"), ED2 ("Recover Decoration") by VAs
Nobunaga the Fool - OP by a VA
Nobunagun - ED by VAs
Noragami - neither
No-Rin - all OPs/EDs by VAs
Onee-chan ga Kita - no OP; ED by a VA
Pupa - OP by VAs
Pupipo! - [note: no ED]
Robot Girls Z - OP and ED by VAs
Saki: Zenkoku-hen - ED3, ED4, ED5 ("Kono Te ga Kiseki..."; eps 3-6, 8) by VAs
Sakura Trick - OP and ED by VAs
Seitokai Yakuindomo 2 - OP and ED by VAs
Silver Spoon (2014) - neither
SoniAni: Super Sonico The Animation - all OPs and EDs by VAs
Space Dandy - neither
Strange+ - OP by a VA; no ED
To Aru Hikuushi e no Koiuta - OP by VAs
Tonari no Seki-kun - OP by a VA
Wake Up, Girls! - both OPs and ED by VAs
Witch Craft Works - ED by VAs
Wizard Barristers: Benmashi Cecil - ED by a VA
Wooser no Sono Higurashi 2 - [note: no OP]
World Conquest Zvezda Plot - OP by a VA
Z/X: Ignition -  OP and ED by a VA


Counting episodes with both an OP and an ED as worth 2 theme songs and episodes with only one of them as worth 1 theme song (and giving full-length episodes and short episodes equal weight), we find that roughly 50% (±5-10%, say) of all theme songs for winter 2014 shows were sung by one or more voice actors of the same show. My sense is that this is more or less what you would expect if you picked a random season from the past 3-4 years.



However, it is important not to be misled by these data, which don't tell the whole story. A number of these theme songs are sung by voice actors who also have an independent musical career. The best example is Sakamoto Maaya (Zvezda OP), but there's a few others in there, too, e.g. "petit milady", which is a new idol unit consisting of Aoi Yuuki and Taketatsu Ayana, initially formed to do the OP for Koiuta (in which they both voice-acted), but which now does other things as well. 

What I'm getting at here is that there's a qualitative difference between Sakamoto Maaya's OP for Zvezda on the one hand, and the ED for Chuunibyou Ren on the other, which is sung by the four female leads of the show, only one of whom (Maaya Uchida) has an independent musical career. 

I basically don't know anything about how anime producers arrange for theme songs to be sung, but I strongly suspect that there are factors that suggest the use of a proper singer from the VA cast (a la Sakamoto Maaya) in some cases, and just glomming all the lead VAs together and having them crank out a song (a la Chuunibyou) in other cases. (And of course, there are still other cases in which you'll bring in an otherwise-uninvolved band or artist, but that goes without saying.)

I haven't gone back and collected any numbers, but in my experience, I have found that shows with primarily-female casts that are targeted at men (e.g. moe shows like Kiniro Mosaic and harem shows like Infinite Stratos) and shows with primarily-male casts that are targeted at women (e.g. otome game adaptations like Diabolik Lovers) are far more likely to follow the glom-all-the-VAs-together-and-have-them-sing model than other shows.