Everything I've seen, examples from SwissRamble and PriceOfFootball, says it's straight line as the norm, indeed the latter has an example of Derby County being criticised by not using straight line - http://priceoffootball.com/football-transfers-and-creative-accounting-protect-me-from-what-i-want/
Though if declining balance was used would suggest that financially we'd be better off as much of the transfer costs would have been offset by now with last season's limited activity meaning limited amounts brought forward this season.