It's at 75% because that's a good compromise. You can go higher but if everything is voice and you're doing no data at all do you really need QoS? The idea of limiting it is to allow routing protocols, administrative access, etc. in the event of severe congestion. If you're using 75% of the pipe for priority traffic, you probably need a bigger pipe or will very soon.
Yes, you can go to 90%. Keep the RTP in a low-latency priority queue. SIP needs little bandwidth and can take buffering and latency but avoid drops with reserved bandwidth. I've run SIP at 2% on busy networks. Make sure your DSCP values are being set as intended and reset to 0 any DSCP from untrusted networks.