This'll do it for you:
/^\d+(?:\.\d)?[05]?$/
The nice thing about this regex is that it will also allow for rationals with only a tenths digit
var re = /^\d+(?:\.\d)?[05]?$/;
re.test(1.77)
=> false
re.test(1)
=> true
re.test(1.05)
=> true
re.test(1.07)
=> false
re.test(1.1)
=> true
re.test(1.10)
=> true
re.test(.5)
=> true
re.test(123.5)
=> true