If Roddick is seeded in the low 20's he won't face a top 5 guy till round 3. If he's high 20's/early 30's he'll face them in the 2nd round.
Small correction, but you're one round short. Seeded players don't meet other seeded players until the 3rd round, and with a higher seeding Mr Decker would avoid a top 4 in the third round.
However, as pointed out already, it is likely Mr Decker won't be seeded at all (Wimbledon uses its own algorithm for the order of the seeds, but the set of 32 seeds is determined by ranking only).