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 $7.71   8.252%   $8.3462   $8.35   $8.35   $8.30  
  Purchase #2 $186.69   6.627%   $199.0619   $199.06   $199.05   $199.10  
  Purchase #3 $204.52   9.847%   $224.6591   $224.66   $224.65   $224.70  
  Purchase #4 $176.58   9.813%   $193.9078   $193.91   $193.90   $193.90  
  Purchase #5 $185.96   3.726%   $192.8889   $192.89   $192.90   $192.90  
  Purchase #6 $60.94   5.556%   $64.3258   $64.33   $64.35   $64.30  
  Purchase #7 $87.85   4.248%   $91.5819   $91.58   $91.60   $91.60  
  7 purchase totals = $910.25       $974.7716   $974.78   $974.80   $974.80