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 $37.31   8.224%   $40.3784   $40.38   $40.40   $40.40  
  Purchase #2 $172.49   4.324%   $179.9485   $179.95   $179.95   $179.90  
  Purchase #3 $29.53   10.112%   $32.5161   $32.52   $32.50   $32.50  
  Purchase #4 $161.93   7.560%   $174.1719   $174.17   $174.15   $174.20  
  Purchase #5 $9.36   12.327%   $10.5138   $10.51   $10.50   $10.50  
  Purchase #6 $80.84   8.555%   $87.7559   $87.76   $87.75   $87.80  
  Purchase #7 $20.70   5.647%   $21.8689   $21.87   $21.85   $21.90  
  7 purchase totals = $512.16       $547.1535   $547.16   $547.10   $547.20