hercules,
We were not doing timed papers until Chrismas break 2013 for independents' entry exams in January 2014. We were not sitting grammars.
I think my DD would have peaked and burned out if we started doing papers that early (but I do realize that you are sitting grammars which are in September, but even then I would leave time practice until summer). She did do a couple of ISEB 11+ papers and was getting high 90s% in those in mid-February, but those are easy and timing was never an issue. Still, some sort of benchmark.
We focused on covering the syllabus - and we had all KS2 covered by this point in time - and starting to do maths problems by type from various papers (e.g., North London Consortium, Dulwich, KCS, Oundle, Perse). It was key for me that she recognized the type and knew the approach and we also did more stretching, scholarship type problems (MGS, Withington Girls, etc). I would throw an (very!!) occasional short paper (the 45min ones), but NOT timed. I wanted to make sure she has a very firm understanding of topics and secure in number bonds, timetables, vertical addition/subtraction, long division/multiplication, fractions - addition, subtraction, conversion between % and decimals, etc.
The we started doing a paper more regularly (e.g. once every week), but again not timed.
Christmas break was all timed staff.
FWIW, DD was getting mid-90% on RGS papers (only silly mistakes) and high 80%/low 90%s on Dulwich College papers (also silly mistakes only) by end of December. Although we had a couple of disasters on some easy papers where she would suddenly get 60% with ONLY silly mistake and make me panic. She got 3 academic scholarships. English was a weaker subject (and dyslexic tendencies).
What you also need to bear in mind is that RGS papers don't have complex, scholarship type problems so you need to diversify a bit if you are going to be sitting for schools that throw that type of stuff into their papers. DD did an Old Palace paper and it had a couple of less straightforward problems - I'd say it was a bit trickier than RGS (but that was a very old paper that our prep sent home, I don't know what they use now).
Hope this helps.