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 $27.78   11.318%   $30.9241   $30.92   $30.90   $30.90  
  Purchase #2 $197.88   6.368%   $210.481   $210.48   $210.50   $210.50  
  Purchase #3 $82.32   10.299%   $90.7981   $90.80   $90.80   $90.80  
  Purchase #4 $142.44   13.377%   $161.4942   $161.49   $161.50   $161.50  
  Purchase #5 $75.33   5.348%   $79.3586   $79.36   $79.35   $79.40  
  Purchase #6 $219.68   12.344%   $246.7973   $246.80   $246.80   $246.80  
  Purchase #7 $160.27   5.528%   $169.1297   $169.13   $169.15   $169.10  
  7 purchase totals = $905.70       $988.9830   $988.98   $989.00   $989.00