Welcome to the forums, umterps98!
As BIslander said, your system will fall back on lossy Dolby Digital 5.1 and the lossy DTS 5.1 core due to the use of an optical cable. Uncompressed 2-channel PCM will be supported optically, though, so you can derive that sonic benefit if the movie has a PCM 2.0 track (such as many concert Blu-rays), as well as stereo CD audio.
Indeed, your Sony STR-DE545 receiver has analog 5.1 channel inputs (I still have my Sony STR-DE845 and remember the whole line very well from back then - even have a closeup photo of it in my
home theater gallery on this site). You can get ALL the beneifts of lossless if you choose a BD player which does internally decode lossless and is equipped with analog multichannel outputs. There will be 7.1 channels of analog outputs on the BD player, so you'll have to make sure that you can set it up to mix the (up to) 4 surround channels (on discs with 6.1 or 7.1 audio) into the 2 surround channels for output to your receiver. This way, you won't miss any information.
Although the lossy (but higher bitrate) audio is good (usually besting lower DVD-release bitrates), I feel it still does not quite hold up against true lossless/uncompressed audio. Vocals/dialog becomes more distinct, and the noticeability and spatical clarity of location-oriented high frequency material (such as environmental sounds) as well as musical details will sound more distinct as lossless/uncompressed sound.
There are those who feel that going analog over HDMI for multichannel sound find they like it better. If you are not up for upgrading your receiver yet, then definitely shoot for a BD player which decodes both Dolby TrueHD and DTS-HD Master Audio and passes them through multichannel analog outputs!
