Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $56.29   6.486%   $59.941   $59.94   $59.95   $59.90  
  Purchase #2 $68.74   7.491%   $73.8893   $73.89   $73.90   $73.90  
  Purchase #3 $61.47   3.493%   $63.6171   $63.62   $63.60   $63.60  
  Purchase #4 $204.53   10.620%   $226.2511   $226.25   $226.25   $226.30  
  Purchase #5 $194.80   4.552%   $203.6673   $203.67   $203.65   $203.70  
  Purchase #6 $207.43   6.719%   $221.3672   $221.37   $221.35   $221.40  
  Purchase #7 $121.74   6.370%   $129.4948   $129.49   $129.50   $129.50  
  7 purchase totals = $915.00       $978.2278   $978.23   $978.20   $978.30