I recommend using your stock plugins until you reach a point where you feel your mixes can't get any better with the tools at your disposal, or where you feel you could benefit from different flavours or colours. Both of these stages of course come after you understand what the plugins do and how and when to use them. The mistake that a lot of inexperienced users make or have made, myself included, is slapping plugins on a track just because 'the pros do it that way', not necessarily because the track needs them.
Cheers,
Sam
This is great advice and 100% agreed with. As for me, I invested in the UAD because I absolutely WANTED the Neve RS88 desk emulation and an EMT250 reverb. Plus the Studer tape emulation..
I try to mix and record with as few plugins as possible.