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 $7.78   7.727%   $8.3812   $8.38   $8.40   $8.40  
  Purchase #2 $18.70   6.653%   $19.9441   $19.94   $19.95   $19.90  
  Purchase #3 $236.87   3.636%   $245.4826   $245.48   $245.50   $245.50  
  Purchase #4 $77.70   9.444%   $85.038   $85.04   $85.05   $85.00  
  Purchase #5 $209.97   13.713%   $238.7632   $238.76   $238.75   $238.80  
  Purchase #6 $132.60   8.536%   $143.9187   $143.92   $143.90   $143.90  
  Purchase #7 $102.37   4.295%   $106.7668   $106.77   $106.75   $106.80  
  7 purchase totals = $785.99       $848.2946   $848.29   $848.30   $848.30