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 $20.61   7.233%   $22.1007   $22.10   $22.10   $22.10  
  Purchase #2 $81.80   5.670%   $86.4381   $86.44   $86.45   $86.40  
  Purchase #3 $121.37   13.309%   $137.5231   $137.52   $137.50   $137.50  
  Purchase #4 $181.84   8.706%   $197.671   $197.67   $197.65   $197.70  
  Purchase #5 $113.68   6.302%   $120.8441   $120.84   $120.85   $120.80  
  Purchase #6 $157.91   8.385%   $171.1508   $171.15   $171.15   $171.20  
  Purchase #7 $233.47   4.277%   $243.4555   $243.46   $243.45   $243.50  
  7 purchase totals = $910.68       $979.1833   $979.18   $979.15   $979.20