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 $218.61   11.167%   $243.0222   $243.02   $243.00   $243.00  
  Purchase #2 $21.57   12.665%   $24.3018   $24.30   $24.30   $24.30  
  Purchase #3 $223.57   5.374%   $235.5847   $235.58   $235.60   $235.60  
  Purchase #4 $74.71   11.246%   $83.1119   $83.11   $83.10   $83.10  
  Purchase #5 $54.41   8.804%   $59.2003   $59.20   $59.20   $59.20  
  Purchase #6 $140.40   9.294%   $153.4488   $153.45   $153.45   $153.40  
  Purchase #7 $52.53   8.147%   $56.8096   $56.81   $56.80   $56.80  
  7 purchase totals = $785.80       $855.4793   $855.47   $855.45   $855.40