I may as well throw in my tuppence. Although credit cards are widely accepted throughout Scotland, I prefer to carry enough cash with me for purchases in small retailers and restaurants, and also to pay the hosts of B&Bs. Visa/MC charges the retailer or B&B owner about 3% of the charges as a service fee, and I've found that most B&B owners, in particular, really appreciate being paid in cash. I've stayed at a few B&Bs where they actually add that 2 1/2 to 3 percent in to the total if you use a card.
If you use a credit card in Scotland (or any foreign country, for that matter), and you don't have a credit card which does not waive foreign transaction fees, you will end up paying about 3% over the charge for each transaction. This can really add up over a long holiday.
Also, if your home bank does not waive foreign ATM fees, you are going to pay a percentage or flat fee each time you use an ATM. If that's the case, and it's a flat fee, you'd want to take out as much as you feel comfortable carrying each time you withdraw money. Some banks waive the foreign ATM fee, but give you a limit on how many transactions you can make before they start charging. Best to check with your home bank before you head OS. Scottish banks do not charge for an ATM transaction, so your only charge would be from your home bank.
With the pound currently trading at $1.22, this is the perfect time to be heading to Scotland.
Best wishes for your holiday.