1 - Talk to your boss about the process of raises.
It's fair to ask to what extent the review influences this. In my experience, the review process is a part of the raise determination system but not the only factor. In most cases, the bosses signing off on self-assessments are the closest bosses to the employees, while the final decision maker on raises is the person who controls the overall budget for a large group of people. So there's always some sort of process between these two forms of management, because raises are as much about throwing the money in the highest value areas for the business as much as for rewarding each employee. To say nothing about the fact that each manager may have their own take on the 1-5 ratings values, so the bigger bosses have to find some way of evening the playing field across teams.
It's usually a messy process and you manager is not likely to want to discuss in detail (it's like making sausage... no one who eats sausage really wants to know how its made). But you should be able to focus the discussion about you and how much this assessment factors in.
2 - Seek generalized feedback on your work
Not just from your boss. And probably not by asking the questions that are giving you trouble -- for example if the assessment says "rate your ability to do foo", you probably don't want to ask a bunch of people in your office "how do you think I do at foo?"... but you might target cases where "foo work" is involved and check in with other folks who have had key responsibilities in these areas and check in with them on how they think these projects and your participation as been in them.
3 - Prepare for the signoff process
Every self-assessment I've ever seen has resulted in a conversation between manager and employee prior to submission. I'm assuming yours is the same.
There have been times when I was certain that it was time for me to get a raise or a promotion... at these times, I've used self-assessments to begin the conversation. When I really think I'm out performing others at my rank, I rate myself noticeably high, and prepare to take the flak from management about whether my high ratings were too high.
It's led to some really useful conversations - about what it would take for promotion or why I wasn't as high as I thought I was. Since I knew I had erred on the side of overconfident, I really didn't mind getting corrected, as long as the correction came with helpful feedback.
It's my personal belief about most assessments that they are there to force employees and managers to really consider work quality and areas of improvement. The result is less important than the effort. Raises & promotions are 1 part the result of good work, but also 1 part the impact of the surrounding enviroment... so they aren't totally under anyone's control.
But assessment and improvement is something you and a manager can work on regardless.
I know some of this stuff is probably old hat if you've been doing it for years already... but when it somes to new questions on an old form, I'd treat them the same way and try to figure out why the company added these questions. Is there some reason they want people to spend time thinking about this stuff? What does it mean for you and why should you care about it? If you can't answer those questions, it's probably time to ask around.