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 $11.76   8.650%   $12.7772   $12.78   $12.80   $12.80  
  Purchase #2 $14.31   8.429%   $15.5162   $15.52   $15.50   $15.50  
  Purchase #3 $173.54   6.491%   $184.8045   $184.80   $184.80   $184.80  
  Purchase #4 $6.59   5.873%   $6.977   $6.98   $7.00   $7.00  
  Purchase #5 $82.96   10.461%   $91.6384   $91.64   $91.65   $91.60  
  Purchase #6 $198.49   12.511%   $223.3231   $223.32   $223.30   $223.30  
  Purchase #7 $134.98   13.526%   $153.2374   $153.24   $153.25   $153.20  
  7 purchase totals = $622.63       $688.2738   $688.28   $688.30   $688.20