The comissions for euros are higher. Typical comissions for USD are max 1% and for euros 4-5%. Instead of carrying euros, you might want to use Visa/Mastercard at ATMs as the cost is quite the same. Most recommendable ATMs with foreign cards are Scotiabank (they permit withdrawals of up 2000 PEN/transaction), Banco Continental and Banco de Credito (also known as BCP). Try to avoid Interbank/Globalnet as they have surcharges for foreign cards. You should be able to find ATMs if every town with at least 50,000 inhabitants.

Don't forget to carry small bills and coins with you as getting change even for 20 soles is sometimes complicated (eg taxis and small stores quite don't have change). You should have a couple of 50-100 euros/USD bills with you just in case you have to change money in small towns like Aguas Calientes or street corners. Especially outside Lima, the lines at banks can be quite long so most likely you will end up using informal cambistas and you might feel unsafe with big bills. Small US bills (max 20 USD) are almost as widely accepted in Peru as soles.