Perceived bitterness and perceived sweetness are related to each other. One way to look at it is in regard to the ratio between gravity and bitterness.
For example - a beer that sits at 50 IBU but starts high and finishes high (say 1080 - 1025)might taste much sweeter than a 1040 30 IBU that finishes at 1010. When designing a recipe it can be a good idea to look at recommended BU/GU ratio ranges (bittering units: gravity units). They are a guide only - anyone can make whatever beer they want and style guidelines can be restrictive if you place too much emphasis on them but if you recognise they are guidelines rather than rules you can use them to your advantage.
Check this link for a rundown:
http://beercolor.netfirms.com/balance.html
Don't worry too much at this point about the formulae and maths - just get a basic understanding of the principle. IBU as a measurement is only useful to a point - that's why some big beers like Russian Imperial Souts and barley Wines are hopped to crazy amounts (besides possibly the preservative qualities of hops helping with ageing).
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