Most anything under 5 mils can be cut with a 45 degree blade. A 60 degree blade should be used for most materials thicker than that but a 45 will probably work OK. Depending. I cut everything with a 45 degree blade. Perhaps I'm missing out on some bit of micro optimization by not using a 60 degree blade on some thicker and/or tougher media. Shrug. A 30 degree blade might be considered for cutting fabric-like media or where a bit more edge rather than just the tip might be a boon. Stuff like Photo-Tex comes to mind.
If you are using a sharp blade with a good tip, it really doesn't matter all that much. The tip is the critical part. If you look at a blade under a loupe or a good magnifying glass, and the tip converges to a point with no visible rounding or has a chip off the tip, it'll most likely cut whatever you're using.
If you're having difficulty cutting something first try changing to a new blade as well as reducing the pen speed. If you're still having problems you then might consider a different angle, but a new blade generally solves most problems. If you think a blade is dull, it probably is. Change it. Moreover, a lot of less than optimum cutting is caused by dust and crud in the blade up/down mechanism. Remove any dust cover and blow it out from time to time. Do not lubricate this mechanism, all that will do is attract more dust.
Down pressure is far more critical than blade angle. If you have the down force properly set you should, in theory, be able to have the blade extended any amount and your rig should cut the media and just dent the backing. It's difficult to achieve this ideal balance so most people, myself included, limit the blade extension as well. Having the end of the blade holder actually riding on the media is not optimum but it's a lot faster to set up than endlessly dicking around with just the pressure.