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 $132.65   4.318%   $138.3778   $138.38   $138.40   $138.40  
  Purchase #2 $129.27   6.362%   $137.4942   $137.49   $137.50   $137.50  
  Purchase #3 $6.85   13.738%   $7.7911   $7.79   $7.80   $7.80  
  Purchase #4 $173.72   3.854%   $180.4152   $180.42   $180.40   $180.40  
  Purchase #5 $224.75   5.575%   $237.2798   $237.28   $237.30   $237.30  
  Purchase #6 $129.28   5.294%   $136.1241   $136.12   $136.10   $136.10  
  Purchase #7 $34.45   12.703%   $38.8262   $38.83   $38.85   $38.80  
  7 purchase totals = $830.97       $876.3084   $876.31   $876.35   $876.30