Governments are ingenious when it comes to sales tax—every jurisdiction seems to have different rates, different exceptions, different refund schemes, and different rules for how to write up taxes on your books. Furthermore, the details are constantly being changed. There's simply no way that MainBoss could ever keep up with all the variations.
Still, if you want an accurate picture of your maintenance expenses, you have to deal with sales tax somehow. One approach is to incorporate the applicable taxes into the costs of each item. If you do this, your MainBoss records will reflect the actual cost of each item, after the taxes are added on. This will help you compare prices if you can purchase items in different jurisdictions (e.g. different states or countries): which place gives you the best price after all taxes are taken into account?
A drawback with this approach is that the after-tax price of an item may be difficult to figure out. For example, if you receive a shipment containing several items, the bill may just give the total sales tax as a single line after the pre-tax total. Then you have to figure out if the tax amount is evenly distributed between the items on the bill or if some items are taxed and others aren't.
An alternative approach is to create a "miscellaneous" entry on each purchase order, in order to cover the sales tax amounts for that purchase order. This is likely easier to write up - if you get a single line giving the sales tax on a bill, you create a single miscellaneous entry for that purchase order. However, it has the effect of separating the tax from the cost of items, which makes it harder to figure out how much you actually pay for the items.