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 $23.97   11.468%   $26.7189   $26.72   $26.70   $26.70  
  Purchase #2 $173.54   12.280%   $194.8507   $194.85   $194.85   $194.90  
  Purchase #3 $170.89   5.672%   $180.5829   $180.58   $180.60   $180.60  
  Purchase #4 $76.99   7.841%   $83.0268   $83.03   $83.05   $83.00  
  Purchase #5 $33.40   3.267%   $34.4912   $34.49   $34.50   $34.50  
  Purchase #6 $197.52   4.153%   $205.723   $205.72   $205.70   $205.70  
  Purchase #7 $88.28   9.339%   $96.5245   $96.52   $96.50   $96.50  
  7 purchase totals = $764.59       $821.9180   $821.91   $821.90   $821.90