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 $25.59   11.514%   $28.5364   $28.54   $28.55   $28.50  
  Purchase #2 $63.83   10.480%   $70.5194   $70.52   $70.50   $70.50  
  Purchase #3 $10.32   7.494%   $11.0934   $11.09   $11.10   $11.10  
  Purchase #4 $188.54   7.351%   $202.3996   $202.40   $202.40   $202.40  
  Purchase #5 $89.79   10.457%   $99.1793   $99.18   $99.20   $99.20  
  Purchase #6 $130.46   3.603%   $135.1605   $135.16   $135.15   $135.20  
  Purchase #7 $83.58   11.338%   $93.0563   $93.06   $93.05   $93.10  
  7 purchase totals = $592.11       $639.9449   $639.95   $639.95   $640.00