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 $143.47   11.370%   $159.7825   $159.78   $159.80   $159.80  
  Purchase #2 $123.57   3.636%   $128.063   $128.06   $128.05   $128.10  
  Purchase #3 $190.98   9.608%   $209.3294   $209.33   $209.35   $209.30  
  Purchase #4 $173.85   9.387%   $190.1693   $190.17   $190.15   $190.20  
  Purchase #5 $73.95   8.131%   $79.9629   $79.96   $79.95   $80.00  
  Purchase #6 $44.81   5.834%   $47.4242   $47.42   $47.40   $47.40  
  Purchase #7 $160.43   8.657%   $174.3184   $174.32   $174.30   $174.30  
  7 purchase totals = $911.06       $989.0497   $989.04   $989.00   $989.10