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 $236.85   12.234%   $265.8262   $265.83   $265.85   $265.80  
  Purchase #2 $115.56   5.428%   $121.8326   $121.83   $121.85   $121.80  
  Purchase #3 $224.61   5.644%   $237.287   $237.29   $237.30   $237.30  
  Purchase #4 $225.51   6.246%   $239.5954   $239.60   $239.60   $239.60  
  Purchase #5 $136.57   9.493%   $149.5346   $149.53   $149.55   $149.50  
  Purchase #6 $246.40   8.575%   $267.5288   $267.53   $267.55   $267.50  
  Purchase #7 $19.96   13.191%   $22.5929   $22.59   $22.60   $22.60  
  7 purchase totals = $1,205.46       $1,304.1975   $1,304.20   $1,304.30   $1,304.10