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 $26.81   11.796%   $29.9725   $29.97   $29.95   $30.00  
  Purchase #2 $154.25   10.360%   $170.2303   $170.23   $170.25   $170.20  
  Purchase #3 $113.83   13.460%   $129.1515   $129.15   $129.15   $129.20  
  Purchase #4 $154.48   7.693%   $166.3641   $166.36   $166.35   $166.40  
  Purchase #5 $152.59   11.405%   $169.9929   $169.99   $170.00   $170.00  
  Purchase #6 $213.59   5.400%   $225.1239   $225.12   $225.10   $225.10  
  Purchase #7 $28.49   4.642%   $29.8125   $29.81   $29.80   $29.80  
  7 purchase totals = $844.04       $920.6477   $920.63   $920.60   $920.70