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 $226.48   11.302%   $252.0768   $252.08   $252.10   $252.10  
  Purchase #2 $20.68   4.457%   $21.6017   $21.60   $21.60   $21.60  
  Purchase #3 $75.50   9.454%   $82.6378   $82.64   $82.65   $82.60  
  Purchase #4 $232.96   9.453%   $254.9817   $254.98   $255.00   $255.00  
  Purchase #5 $154.44   10.834%   $171.172   $171.17   $171.15   $171.20  
  Purchase #6 $239.89   9.620%   $262.9674   $262.97   $262.95   $263.00  
  Purchase #7 $242.65   4.534%   $253.6518   $253.65   $253.65   $253.70  
  7 purchase totals = $1,192.60       $1,299.0892   $1,299.09   $1,299.10   $1,299.20