[bely832new]

Salaam!

According to the shafi'is, apart from one exception, you can pray any of the nawaafil at any time if they have a sabab (i.e. cause).

E.g. You can pray tahiyyatul masjid when if you enter the mosque between Asr and Maghrib. But you can't pray 2 rak'ah just without any reason.

Allah knows best. I believe hanafis are quite strict i.e. you can't pray nawaafil at all (http://spa.qibla.com/issue_view.asp?...D=3292&CATE=98). Other brothers can confirm.