Jenny,
Even though you're travelling in April (just prior to the start of the "spring shoulder season"), I'd suggest booking in advance where possible, especially since you stated that "I'll know my time frame".
One advantage of pre-booking either Hostels or Hotels is that I don't have to waste any time trying to find a place to sleep when I arrive in a new (and unfamiliar) city, which can waste valuable touring time. In addition, that provides me with choices in both the area of the city I want to use as a "base", and in the case of Hotels the price range / rating (ie: two star, three star or whatever).
In addition to Hostel World, you might find it useful to have a look at This Website. These are a "chain" of Hostels and I've found that the properties are fairly consistent from one property to the next. They usually provide breakfast as well, but how "substantial" that is varies a bit between locations. One final point is that having a membership in these (very reasonable cost) provides not only a better rate but preferred booking over non-members.
If you need any Hostel recommendations for specific cities, post another note. I'm sure the group here will have lots of ideas.
Happy travels!