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 $77.54   8.464%   $84.103   $84.10   $84.10   $84.10  
  Purchase #2 $33.44   6.437%   $35.5925   $35.59   $35.60   $35.60  
  Purchase #3 $124.91   7.447%   $134.212   $134.21   $134.20   $134.20  
  Purchase #4 $209.27   9.848%   $229.8789   $229.88   $229.90   $229.90  
  Purchase #5 $107.38   13.389%   $121.7571   $121.76   $121.75   $121.80  
  Purchase #6 $57.37   3.814%   $59.5581   $59.56   $59.55   $59.60  
  Purchase #7 $172.76   5.806%   $182.7904   $182.79   $182.80   $182.80  
  7 purchase totals = $782.67       $847.8920   $847.89   $847.90   $848.00