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 $246.44   10.873%   $273.2354   $273.24   $273.25   $273.20  
  Purchase #2 $154.54   9.157%   $168.6912   $168.69   $168.70   $168.70  
  Purchase #3 $134.95   10.838%   $149.5759   $149.58   $149.60   $149.60  
  Purchase #4 $244.62   12.216%   $274.5028   $274.50   $274.50   $274.50  
  Purchase #5 $131.37   9.213%   $143.4731   $143.47   $143.45   $143.50  
  Purchase #6 $221.55   4.238%   $230.9393   $230.94   $230.95   $230.90  
  Purchase #7 $73.54   6.767%   $78.5165   $78.52   $78.50   $78.50  
  7 purchase totals = $1,207.01       $1,318.9342   $1,318.94   $1,318.95   $1,318.90