It depends on what you mean by IceGuide being reliable. It's actually pretty good at getting the scheduled times correct for program start and end times. After that, the TV networks decide not to actually start and end their programs to schedule. On some programs IceTV tries to compensate for that behaviour of the networks. Sometimes they get that right, sometimes not. When they predict an actual start time, but the network in fact runs the program earlier than that, then without pre-padding you'll miss the beginning of the program.
IceTV does not use published guide data for the commercial broadcasters. They assemble their own guide for them, mainly based on predicting the broadcaster's behaviour.
There are some circumstances where padding can't be applied fully, because that would need more simultaneous recordings that the PVR is capable of. For example, if you have a limit of two simultaneous recordings on the PVR, and you record ABC1 20:00-21:00, SBS1 20:00-21:00, Nine 21:00-22:00 and Seven 21:00-22:00, the padding that should be applied pre- and post- 21:00 can't be applied in the expected way. Only having more tuners (e.g. another recorder) can you be sure that padding can be applied.
I run our two Beyonwizes with 5 min pre-padding and 25 min post-padding, I'm careful to try to avoid padding conflicts like that, and when the padding can be applied I only very occasionally miss parts of programs.
For the programs where you are missing parts of the recording, is padding being properly applied? If not, then that's probably more of a problem with the PVR, either bugs or inherent limitations (as in the example above), than a problem with IceTV itself.
Are you sure whether padding is actually being applied to the programs? Do their actual recording start and end times reflect the amount of padding that should be applied.
Unfortunately, IceTV isn't nearly as "hands off" as certain IceTV slogans might lead you to believe, and yes, sometimes they get it wrong.
