That's a common assumption, and it certainly works, but there's no real "rule" for or against it afaik.
(This is copied from 4th ed pdf, which is certainly a precedent to 5th.)
Level
Most spells are assigned a level, which is usually a multiple of five. A 5th level spell is very weak, a 15th level spell is of moderate power, and a 30th level spell is quite powerful. Some spells are General spells (abbreviated to Gen), which means that they may be learned at any level of difficulty —the higher the level, the more powerful the spell. You may even learn General spells at levels that are not multiples of 5.
This specifies that General spells can, but never addresses Ritual spells per se, one way or the other. The word "usually" is not specifically tied to only General spells, tho' it would have been easy to do so.
Spells are rated by levels, according to their power. Spells of greater power have a greater level. Magi refer to spell levels by using the word magnitude (magi do not use the term level), and one order of magnitude encompasses five levels of spells. Spells of the first magnitude, therefore, are those spells of levels 1-5.
Pretty clear here.
The storyguide may always intervene and declare that a certain combination of range, duration, target, and effect warrants a higher or lower level than that described by the guidelines and the system above.
This also uses the term "Level", not "Magnitudes" or "multiples of 5", even tho' specifically speaking only of "range, duration, target, and effect."
The categories described here were built into the structure of Hermetic magic by Bonisagus. All spontaneous spells must conform to these requirements (the magus is making the spell up on the fly—he cannot also push the limits of magic theory). Formulaic spells, on the other hand, can be invented with ranges, durations or targets that are not listed here. This is usually slightly more difficult than if the closest category were used, but is largely left to storyguide interpretation.
Again, no mandate that ritual spells must be measured in full magnitudes, or only multiples of 5. What does "slightly more difficult" mean to you? Each GM must answer that themselves- that much is even spelled out.
95% of the time, the authors refer to Level, not Magnitude, and I believe there's a reason for that.
Maybe they left if intentionally vague. But vague it is, and as such completely open to interpretation. Don't know if 5th re-emphasizes one interpretation over the other, or what.