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 $174.84   9.178%   $190.8868   $190.89   $190.90   $190.90  
  Purchase #2 $125.26   9.782%   $137.5129   $137.51   $137.50   $137.50  
  Purchase #3 $77.47   10.146%   $85.3301   $85.33   $85.35   $85.30  
  Purchase #4 $185.37   4.841%   $194.3438   $194.34   $194.35   $194.30  
  Purchase #5 $129.25   4.340%   $134.8595   $134.86   $134.85   $134.90  
  Purchase #6 $23.31   10.198%   $25.6872   $25.69   $25.70   $25.70  
  Purchase #7 $76.62   6.270%   $81.4241   $81.42   $81.40   $81.40  
  7 purchase totals = $792.12       $850.0444   $850.04   $850.05   $850.00