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 $221.42   13.787%   $251.9472   $251.95   $251.95   $251.90  
  Purchase #2 $149.96   12.773%   $169.1144   $169.11   $169.10   $169.10  
  Purchase #3 $61.63   5.841%   $65.2298   $65.23   $65.25   $65.20  
  Purchase #4 $169.44   13.476%   $192.2737   $192.27   $192.25   $192.30  
  Purchase #5 $118.89   6.441%   $126.5477   $126.55   $126.55   $126.50  
  Purchase #6 $202.82   10.551%   $224.2195   $224.22   $224.20   $224.20  
  Purchase #7 $8.87   3.805%   $9.2075   $9.21   $9.20   $9.20  
  7 purchase totals = $933.03       $1,038.5398   $1,038.54   $1,038.50   $1,038.40