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 $8.57   10.435%   $9.4643   $9.46   $9.45   $9.50  
  Purchase #2 $11.67   11.168%   $12.9733   $12.97   $12.95   $13.00  
  Purchase #3 $164.37   9.380%   $179.7879   $179.79   $179.80   $179.80  
  Purchase #4 $40.84   13.544%   $46.3714   $46.37   $46.35   $46.40  
  Purchase #5 $25.87   12.411%   $29.0807   $29.08   $29.10   $29.10  
  Purchase #6 $57.61   3.680%   $59.73   $59.73   $59.75   $59.70  
  Purchase #7 $73.89   12.421%   $83.0679   $83.07   $83.05   $83.10  
  7 purchase totals = $382.82       $420.4755   $420.47   $420.45   $420.60