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I decided to build my digital dvd and blu-ray collection from scratch and I want to do it properly from the start so I'd like to know if I'm doing something wrong when selecting what audio track to rip and what not.

Currently I have 3.1 setup (front, center, and sub) and avr receiver capable of handling dts-hd audio. I'll expand the setup to 5.1 soon for sure (before Christmas), perhaps to 7.1 some day.

If a movie had these audio tracks:
dts-hd ma 7.1, dts-hd ma 5.1, dts 5.1 and DD 5.1

which should I rip and which not? Should I rip both dts-hd ma 7.1 and dts-hd ma 5.1 or just one of them? If one has 5.1 setup, is it better to use 5.1 audio track than 7.1 audio track? Does it matter?

In general you want to retain the highest quality you can for your viewing scenario. If you don't know what that scenario looks like, or want to future proof, and you don't care about disk space too much, save the highest quality tracks available that your playback supports as input, and let your player or hardware downmix or downscale to play on the target device, whether that's stereo AAC or 7.1 DTS.