I generally let them build up then transfer to SPG when I've made an SPG booking so I still have the flexibility to use them for something else like a Velocity bonus offer. Transfer times to SPG are getting better - taking about 3 weeks for me last few times.
I think DC's month in arrears points method is a little past best practice now (esp since Amex have been posting points in real time for a long time) but a quirk I've learnt to live with.
was impressed by my transfer this month.... 9 days from Diners to SPG.
FYI ... DC say DC-SPG transfers need to be in 3000 DC Rewards Point lots.
I recently did an *online* DC-SPG transfer in a non-3000 DC Reward Point sum and it went through fine.