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 $248.41   8.470%   $269.4503   $269.45   $269.45   $269.50  
  Purchase #2 $92.64   3.222%   $95.6249   $95.62   $95.60   $95.60  
  Purchase #3 $121.25   8.465%   $131.5138   $131.51   $131.50   $131.50  
  Purchase #4 $228.61   7.821%   $246.4896   $246.49   $246.50   $246.50  
  Purchase #5 $233.32   5.148%   $245.3313   $245.33   $245.35   $245.30  
  Purchase #6 $245.73   7.137%   $263.2678   $263.27   $263.25   $263.30  
  Purchase #7 $61.69   7.692%   $66.4352   $66.44   $66.45   $66.40  
  7 purchase totals = $1,231.65       $1,318.1129   $1,318.11   $1,318.10   $1,318.10