Low-profile filters?

Some filters claim "slim" profile to accommodate wide-angle lenses. However, with that claim manufacturers often jack up the price. Case in point: B+W regular ~$40 vs. B+W slim ~$80. Anyone has an experience if it really matters if filters are not stacked? I plan to buy a 21mm equivalent lens - would the slim filter profile matter at all for filters I consider, given I almost never put two filters simultaneously on the glass?