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 $250.98   13.662%   $285.2689   $285.27   $285.25   $285.30  
  Purchase #2 $235.55   11.496%   $262.6288   $262.63   $262.65   $262.60  
  Purchase #3 $125.52   8.691%   $136.4289   $136.43   $136.45   $136.40  
  Purchase #4 $90.73   5.332%   $95.5677   $95.57   $95.55   $95.60  
  Purchase #5 $5.73   9.476%   $6.273   $6.27   $6.25   $6.30  
  Purchase #6 $50.94   12.417%   $57.2652   $57.27   $57.25   $57.30  
  Purchase #7 $75.40   6.126%   $80.019   $80.02   $80.00   $80.00  
  7 purchase totals = $834.85       $923.4515   $923.46   $923.40   $923.50