You're not going to be a cutting-edge guy by using HTML5 and CSS3. You'll be a cutting-edge guy by creating a light and fast cutting-edge site that uses the latest (and current) standards, goes one mile further than the average designer to deliver a top-notch, flexible, accessible and user-friendly site for as many people as possible, regardless of the device used to access a website.
If your design is great, then missing rounded corners and shadows for users on legacy browsers won't have much of a significance. Besides, I strongly doubt that a casual user would care about such trivialities. Visitors come to your site for the content, not for the design.
I've mostly abandoned using images for rounded corners and drop shadows, the latter mainly because, a) it rarely does anything useful to a design in terms of aesthetics and should only be used (IMO) to make a content element stand out, and b) using CSS3 for drop-shadows can be a true resource hog.
Border-radius is even worse in terms of performance, so I use it very sparingly as well. In fact, I think in some instances (rounded top left/top right corners nav tabs) an image sprite might be less resource intense.