The Practice (Series)

It's not necessary, the pilot of Boston Legal catches ya' up pretty quick on who the characters are you're following, the only ones being carried over from The Practice being four characters. So other than Alan and Tara (introduced at the start of The Practice Season 8), no other regulars are carried over, while Denny and Sally had only showed up for that final year's last half-dozen eps (thus considered 'guest characters' before spinning off). There is at least one other guest character from The Practice's final season who becomes a recurring character in Boston Legal, played by Betty White, though really just due to knowing the Alan character since he was a child.

None of the 'original' regulars from The Practice show up in Boston Legal, so the last time Alan or any of the other carry-overs see them in Season 8 really is the last onscreen time. There's actually very little reference to the events of The Practice, so it's not like you'd miss a glaring continuity reference (like say when watching Buffy's Angel spinoff), only a handful of subtle moments. Hell even if you do wanna follow the backstory of the characters that get spun-off, Season 8 is the only relevant Practice season in regards to them and can be watched without viewing the prior seven seasons IMO.

I would strongly suggest at least nabbing The Practice Season 8, if anything just to have a bit more of the amazing Alan/Denny characters than only five seasons offer. You'll see how Alan Shore comes to Denny's firm, what paves the way to them becoming best friends, the start of Alan's relationship with Sally, and the underlining sensual tension between Shore and Tara. Boston Legal Season 1 is then a kinda transitional season in terms of tone, whereas the rest of the series is a pretty laid-back dramedy and The Practice is a full-fledged drama, the first year of this spinoff blends both concepts pretty well IMHO.

Superman & The [Red-Blue] Blur = The Man of Steel

The Practice itself is a great show and more dramatic, less comical than BL.

There are some shocking scenes and crazy cases.