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 $17.50   5.212%   $18.4121   $18.41   $18.40   $18.40  
  Purchase #2 $99.65   9.599%   $109.2154   $109.22   $109.20   $109.20  
  Purchase #3 $224.67   4.566%   $234.9284   $234.93   $234.95   $234.90  
  Purchase #4 $237.59   3.255%   $245.3236   $245.32   $245.30   $245.30  
  Purchase #5 $185.64   10.406%   $204.9577   $204.96   $204.95   $205.00  
  Purchase #6 $74.33   11.373%   $82.7836   $82.78   $82.80   $82.80  
  Purchase #7 $13.34   6.605%   $14.2211   $14.22   $14.20   $14.20  
  7 purchase totals = $852.72       $909.8419   $909.84   $909.80   $909.80