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 $245.89   9.742%   $269.8446   $269.84   $269.85   $269.80  
  Purchase #2 $25.61   4.494%   $26.7609   $26.76   $26.75   $26.80  
  Purchase #3 $171.89   3.207%   $177.4025   $177.40   $177.40   $177.40  
  Purchase #4 $137.86   5.247%   $145.0935   $145.09   $145.10   $145.10  
  Purchase #5 $115.54   7.663%   $124.3938   $124.39   $124.40   $124.40  
  Purchase #6 $15.40   8.273%   $16.674   $16.67   $16.65   $16.70  
  Purchase #7 $220.56   4.667%   $230.8535   $230.85   $230.85   $230.90  
  7 purchase totals = $932.75       $991.0228   $991.00   $991.00   $991.10