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 $57.93   9.115%   $63.2103   $63.21   $63.20   $63.20  
  Purchase #2 $103.43   4.759%   $108.3522   $108.35   $108.35   $108.40  
  Purchase #3 $188.89   8.578%   $205.093   $205.09   $205.10   $205.10  
  Purchase #4 $73.42   6.147%   $77.9331   $77.93   $77.95   $77.90  
  Purchase #5 $33.72   10.116%   $37.1311   $37.13   $37.15   $37.10  
  Purchase #6 $245.95   3.203%   $253.8278   $253.83   $253.85   $253.80  
  Purchase #7 $84.61   8.406%   $91.7223   $91.72   $91.70   $91.70  
  7 purchase totals = $787.95       $837.2698   $837.26   $837.30   $837.20