Hello John
Yes, you’re right - it is high season and that means popular places, especially Edinburgh because of the Festivals, get very busy. You could risk taking pot luck especially in the north and west but overall I’d say book ahead. In Edinburgh itself there’s a huge choice though I note that Violet Bank House is consistently extramiled. (Check most of these suggestions by using the a-z on this site.)
Let’s face it, Edinburgh at peak time is, well, completely manic. If you want somewhere a bit more peaceful but within easy reach of lots of interesting places in central Scotland, then Tigh na Leigh in Alyth in Angus is great – comfortable, friendly and good food. South of the city, not far from the Border, the Buccleuch Arms at Moffat has great service and food. In Perthshire the Ardeonaig Hotel is sensational (but at a price!) For a flavour of the remoter Highlands, then good things are said about Meall mo Chridhe - Little Hill of My Heart by Applecross. We’ve also stayed at Berabhaigh in Broadford, Isle of Skye, with the MacMillans, a local family, and found them very friendly and knowledgeable.
In general, VisitScotland’s grading scheme is reliable as well – the more stars, the better the quality whatever the category of accommodation. I’d rather stay in a four star b & b than a two star hotel any time!