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 $236.54   13.841%   $269.2795   $269.28   $269.30   $269.30  
  Purchase #2 $169.71   4.276%   $176.9668   $176.97   $176.95   $177.00  
  Purchase #3 $44.64   7.580%   $48.0237   $48.02   $48.00   $48.00  
  Purchase #4 $107.98   5.157%   $113.5485   $113.55   $113.55   $113.50  
  Purchase #5 $132.61   3.221%   $136.8814   $136.88   $136.90   $136.90  
  Purchase #6 $199.72   12.700%   $225.0844   $225.08   $225.10   $225.10  
  Purchase #7 $224.70   12.862%   $253.6009   $253.60   $253.60   $253.60  
  7 purchase totals = $1,115.90       $1,223.3852   $1,223.38   $1,223.40   $1,223.40