That's also a useful tool Ash, although it won't deal with URLs that that have parameters in such as question marks.

What are you actually doing, redesigning a site and URLs have changed?

Although regardless, the process is normally the same. As soon as you have launched your new site, made live your new page or whatever is you are redirecting, set the redirects at the same time. There is no reason to delay them and again I really wouldn't bother with doing the redirects as 302's to start with. Just set the 301's, then check them all after, then double check. Even if some are incorrect after implementing, its no biggie. Simply edit it again so its correct .

The best thing to do is prepare all your redirects before I hand. I like to get organised and use a spread sheet with two columns, old URLs in one column, new URLs in the 2nd column. This way you can easily map them out.

Also to speed up the process of actually coding the redirects, I use this tool, it saves a huge amount of time: HtAccess 301 Redirect Generator Tool .

Like you mentioned, the reasoning behind doing 302 redirects initially is in case the redirect has been set incorrectly, giving you time to check them before Google re-crawls and updates the index. As 302 redirects are temporary, after setting the redirect the old page will usually stay in Google's index indefinitely.

However I do many 301 redirects every week and I have never taken this approach; I simply set the 301 redirect, then check everything is correct straight after. If you have hundreds of redirects to do you can check them more efficiently using the Screaming Frog web crawler. You can import your list of pages that should have been redirected in a txt file, then Screaming frog will crawl them telling you what response code they are sending and where they are redirecting too.

Even if you set an incorrect 301 redirect, it can takes weeks to months before Google updates the index, so you shouldn't really need to worry about sending the 'juice' to the incorrect page; although I suppose it depends on how long until you check if you have done those redirects correctly!

There could also be adverse issues. Say you were doing a site migration and you 302 redirected every URL on your site to other URLs, I don't know how Google would react to that, but it could be rather bad!

As the saying goes 'Measure Twice, Cut Once'!