firstly - welcome to Lumberjocks!
now for the questions. and I'll start with the 2nd one since the answer to the first question relates to it:
when making panels , or anything for that matter, you cannot glue end grain to end grain - only long grain to long grain, so you usually want to use as long lumber as you can get that will cover the entire length of the part (mostly - some cases are different). unless you want to stager them - but in the case of panels, I wouldn't think you'd want that.
as for the first question - keep the lumber in it's longest form until you need to use it - then mill it per the project requirement - get the longest parts you can from it. I wouldn't just cut it all down to "generic sizes" since in the long run you might need something longer.
now for the questions. and I'll start with the 2nd one since the answer to the first question relates to it:
when making panels , or anything for that matter, you cannot glue end grain to end grain - only long grain to long grain, so you usually want to use as long lumber as you can get that will cover the entire length of the part (mostly - some cases are different). unless you want to stager them - but in the case of panels, I wouldn't think you'd want that.
as for the first question - keep the lumber in it's longest form until you need to use it - then mill it per the project requirement - get the longest parts you can from it. I wouldn't just cut it all down to "generic sizes" since in the long run you might need something longer.