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 $129.38   10.615%   $143.1137   $143.11   $143.10   $143.10  
  Purchase #2 $111.77   10.154%   $123.1191   $123.12   $123.10   $123.10  
  Purchase #3 $232.84   4.441%   $243.1804   $243.18   $243.20   $243.20  
  Purchase #4 $195.63   11.791%   $218.6967   $218.70   $218.70   $218.70  
  Purchase #5 $103.58   8.160%   $112.0321   $112.03   $112.05   $112.00  
  Purchase #6 $39.61   6.770%   $42.2916   $42.29   $42.30   $42.30  
  Purchase #7 $97.55   10.380%   $107.6757   $107.68   $107.70   $107.70  
  7 purchase totals = $910.36       $990.1093   $990.11   $990.15   $990.10