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 $30.61   6.311%   $32.5418   $32.54   $32.55   $32.50  
  Purchase #2 $11.50   9.640%   $12.6086   $12.61   $12.60   $12.60  
  Purchase #3 $74.88   12.678%   $84.3733   $84.37   $84.35   $84.40  
  Purchase #4 $110.59   5.497%   $116.6691   $116.67   $116.65   $116.70  
  Purchase #5 $107.50   11.319%   $119.6679   $119.67   $119.65   $119.70  
  Purchase #6 $24.35   3.381%   $25.1733   $25.17   $25.15   $25.20  
  Purchase #7 $192.58   4.838%   $201.897   $201.90   $201.90   $201.90  
  7 purchase totals = $552.01       $592.9310   $592.93   $592.85   $593.00