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 $240.78   12.565%   $271.034   $271.03   $271.05   $271.00  
  Purchase #2 $184.64   4.609%   $193.1501   $193.15   $193.15   $193.20  
  Purchase #3 $31.91   12.849%   $36.0101   $36.01   $36.00   $36.00  
  Purchase #4 $100.34   5.230%   $105.5878   $105.59   $105.60   $105.60  
  Purchase #5 $168.58   5.406%   $177.6934   $177.69   $177.70   $177.70  
  Purchase #6 $41.63   11.344%   $46.3525   $46.35   $46.35   $46.40  
  Purchase #7 $149.25   11.517%   $166.4391   $166.44   $166.45   $166.40  
  7 purchase totals = $917.13       $996.2670   $996.26   $996.30   $996.30