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 $17.99   11.854%   $20.1225   $20.12   $20.10   $20.10  
  Purchase #2 $114.55   13.595%   $130.1231   $130.12   $130.10   $130.10  
  Purchase #3 $121.33   13.595%   $137.8248   $137.82   $137.80   $137.80  
  Purchase #4 $170.32   12.857%   $192.218   $192.22   $192.20   $192.20  
  Purchase #5 $183.87   11.563%   $205.1309   $205.13   $205.15   $205.10  
  Purchase #6 $23.80   8.778%   $25.8892   $25.89   $25.90   $25.90  
  Purchase #7 $216.75   5.300%   $228.2378   $228.24   $228.25   $228.20  
  7 purchase totals = $848.61       $939.5463   $939.54   $939.50   $939.40