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 $128.35   6.847%   $137.1381   $137.14   $137.15   $137.10  
  Purchase #2 $12.34   5.280%   $12.9916   $12.99   $13.00   $13.00  
  Purchase #3 $84.48   13.514%   $95.8966   $95.90   $95.90   $95.90  
  Purchase #4 $99.40   3.355%   $102.7349   $102.73   $102.75   $102.70  
  Purchase #5 $119.83   7.523%   $128.8448   $128.84   $128.85   $128.80  
  Purchase #6 $22.37   11.704%   $24.9882   $24.99   $25.00   $25.00  
  Purchase #7 $218.56   13.149%   $247.2985   $247.30   $247.30   $247.30  
  7 purchase totals = $685.33       $749.8927   $749.89   $749.95   $749.80