It generally depends on the powder used if it will effect accuracy, some powders don't like being compressed.
Their is a general consensus that 'Ball' powders don't like compression, but I have never found this to be true.
Most large cases have the powder slightly compressed by the bullet by design, it's par and parcel with slow powder taking up nearly all of the internal space of the case.
If a load density is 100% or higher, it will be compressed considerably when the bullet is seated, but it will not have any detrimental affects on how the powder performs if it is slow enough, fast powders, if you can get enough of it in a case to compress, will behave erratically, so it's generally recommended to use slow powders that fill the case a minimum of 75%.
Even loads that are of a load density of 75%-80% can be compressed if a long for calibre bullet is used.
I would not be concerned with compressed loads, unless they are getting into really heavy compression, say above 115% density.
As to accuracy, a slightly compressed load will normally have better ignition characteristics, therefore improving accuracy due to 'packing scheme', the powder is not able to 'slosh' around, and will sit against the flash hole with regularity, shot-shot.
Hope this helps.
Cheers.
MagnumManiac.
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