Plant, unless you book with one of the luxury lines, you will find that gratuities are part and parcel of cruising. Most lines add them as a per day cost (varying from about $A11.50 per day per person though I have seen some of the UK lines less than this). This amount is pooled and shared with the staff who work to provide you with a fabulous cruise experience but who you may not meet. Note that ship's crew and officers are different and not included.
Many lines have the option to pre-pay your gratuities and we usually do this. Also you can sometimes get a deal where grats are included. Working out the cost will help you decide if it is worth pre-paying or taking up a free offer if available. For example I would take free grats over $150 OBC as they will be worth much more.
It is your choice if you would like at tip any staff individually at the end of the cruise and is not necessary if you have left your auto gratuities in place or pre-paid them.
Depending who you cruise with, you may be able to book through a US travel agent who may offer more 'perks' but not all cruise lines allow this. We usually book direct with the cruise line itself online. What we do is look at the website of a couple on online TAs and do a search for the location and time, then compare this with the cruise line itself.
I would think that if you are looking at one of the mainstream cruise lines the peak time is in the the northern summer or late spring/early autumn.
Some of the trip reports although land based have visited some of the towns up the Norwegian coast and you might get some ideas from them.
We have only cruised out of Tromso in Norway on the way to Svalbard and Iceland so have not really done the itinerary you are after.
Cruise Critic, a sister site of Tripadvisor, will have information for you. It is free to sign up. The link to Northern European ports is
here