Edit:
tldr: 91% DOES NOT lose its percentage as it evaporates (you can reuse it almost forever), 70% WILL lose its percentage and you will have to buy more.
The content below is probably why Formlabs recommends 90% and up. In regards to your filter, 5 microns for IPA should be just fine, but may be overkill for the resin (don’t quote me). Due to the viscosity of the resin, running resin through the 5 micron might have you waiting. Also, the alcohol does evaporate. Every time you open the container, there’s a tad bit less than you had before.
Source: http://www.quora.com/When-is-70-isopropyl-rubbing-alcohol-better-than-91
Content:
91% Isopropyl Alcohol [and above]
More flammable
Faster, better drying agent
Evaporation will not change alcohol percentage level [1]
Lower water content and faster drying kinder to items prone to water damage
Stronger odor
70% Isopropyl Alcohol
Equally effective or more as an antiseptic [2]
Less expensive [1]
Safer flammability level
Alcohol % levels will erode if left uncapped [1]
[1] Isopropyl alcohol is azeotropic at a 91% mixture level, which means that it will evaporate and boil away exactly as it exists, and it is also at the highest isopropyl percentage that is possible through distillation. The 70% variety is therefore less expensive because it requires less distillation, and it can in fact also be created by merely adding water to the 91% variety. Any rubbing alcohol with less than the azeotropic percentage of 91% will evaporate a disproportionately large percentage of its alcohol, leaving the remainder with less and, potentially, eventually leaving only water behind. Somewhat counterintuitive is that at levels over 91%, achievable through methods other than distillation, the evaporate will contain a lower alcohol percentage than is contained within the liquid that it evaporates from.
http://www.chemguide.co.uk/physical/phaseeqia/nonideal.html
[2] The CDC no longer considers alcohols to be high-level disinfectants for institutional use “because of their inability to inactivate bacterial spores and because of the inability of isopropyl alcohol to inactivate hydrophilic viruses (i.e. poliovirus, coxsackie virus)”, but it does recommend it for home use, along with hydrogen peroxide and bleach, and states that a 70% ethanol solution has been demonstrated to be the “the most effective concentration for killing the tissue phase of Cryptococcus neoformans, Blastomyces dermatitidis, Coccidioides immitis, and Histoplasma capsulatum and the culture phases of the latter three organisms aerosolized onto various surfaces”. It does not address isopropyl solution levels beyond saying that “isopropyl alcohol (isopropanol) was slightly more bactericidal than ethyl alcohol for E. coli and S. aureus” and “cidal activity drops sharply when diluted below 50% concentration, and the optimum bactericidal concentration is 60%–90% solutions in water” (this last comment relates to both ethanol and isopropyl).
The 70% formulation, by evaporating more slowly, may also actually assist in its disinfectant role, although this comment is not coming from the provided CDC document links but is instead deduced from them. The slower evaporation rate means that, in some settings, exposure to the alcohol will be for a longer period of time. Length of exposure is a key factor in determining the effectiveness of a disinfectant, as the CDC documents do repeatedly reference.
http://www.cdc.gov/hicpac/pdf/guidelines/disinfection_nov_2008.pdf pages 12, 27
http://www.cdc.gov/hicpac/Disinfection_Sterilization/6_0disinfection.html