I was searching this board for 'picture frames' and came across this question and thought I'd add an answer which might be of interest.
The 'correct' way (i.e. the way every professional frame does it) to trim moulding for picture frames is to use a Morso mitre guillotine (or a copy).
This makes perfectly smooth cuts at any angle (but usually 45 degrees) and has a measurement system that uses a vernier-type scale that allows you to dial in the size of the glass (plus 3-4mm as the glass shouldn't be tight up against the frame), the depth of the rebate in the moulding and then it give cuts accurate to the mm.
You place the moulding against the 45 degree stop (having used one side of the guillotine to make your initial angled cut) read the depth of the rebate from the vertical scale, then set this reading on the top horizontal scale against the desired glass size on the bottom horizontal scale.
So if the rebate depth is 25 mm and the glass size is 350mm, slide the top scale so that 25 is against 350 and then use the foot pedal to tike bites from the moulding. There is a hand control to move the blades in after each cut and 7-8 cuts completes the cut.
I'm in no way suggesting you buy one of these, but thought you might be interested.
You can also get a table-top version with a hand lever, and I believe these are popular with kitchen fitters etc who have to cut a lot of mouldings at 45 degrees.