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 $43.39   13.562%   $49.2746   $49.27   $49.25   $49.30  
  Purchase #2 $155.98   8.551%   $169.3178   $169.32   $169.30   $169.30  
  Purchase #3 $59.77   10.243%   $65.8922   $65.89   $65.90   $65.90  
  Purchase #4 $205.34   3.579%   $212.6891   $212.69   $212.70   $212.70  
  Purchase #5 $185.38   10.122%   $204.1442   $204.14   $204.15   $204.10  
  Purchase #6 $69.70   7.231%   $74.74   $74.74   $74.75   $74.70  
  Purchase #7 $60.33   5.220%   $63.4792   $63.48   $63.50   $63.50  
  7 purchase totals = $779.89       $839.5371   $839.53   $839.55   $839.50