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 $154.84   8.247%   $167.6097   $167.61   $167.60   $167.60  
  Purchase #2 $5.30   13.290%   $6.0044   $6.00   $6.00   $6.00  
  Purchase #3 $236.48   6.248%   $251.2553   $251.26   $251.25   $251.30  
  Purchase #4 $72.78   12.788%   $82.0871   $82.09   $82.10   $82.10  
  Purchase #5 $22.34   6.700%   $23.8368   $23.84   $23.85   $23.80  
  Purchase #6 $185.32   11.219%   $206.1111   $206.11   $206.10   $206.10  
  Purchase #7 $232.48   10.368%   $256.5835   $256.58   $256.60   $256.60  
  7 purchase totals = $909.54       $993.4879   $993.49   $993.50   $993.50