Starting with a temp of 148 F and mashing for 90 minutes I would think that you would already have a wort with high apparent attention and it would be too late to retain much residual extract i.e. Elvis has already left the building. Comments please!Yup.
Mashing out tends to produce a slightly higher gravity (~2% boost) as not all the starch gelatinizes at standard single infusion mash temperatures. If you gradually increase mash temp to 168 F, you don't need to rest at 168. Similarly, if you increase wort temperature to 168 F, you don't need to rest afterwards.