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 $246.49   12.622%   $277.602   $277.60   $277.60   $277.60  
  Purchase #2 $63.89   6.197%   $67.8493   $67.85   $67.85   $67.80  
  Purchase #3 $14.95   9.214%   $16.3275   $16.33   $16.35   $16.30  
  Purchase #4 $113.28   5.615%   $119.6407   $119.64   $119.65   $119.60  
  Purchase #5 $226.41   12.241%   $254.1248   $254.12   $254.10   $254.10  
  Purchase #6 $12.39   13.174%   $14.0223   $14.02   $14.00   $14.00  
  Purchase #7 $90.64   8.440%   $98.29   $98.29   $98.30   $98.30  
  7 purchase totals = $768.05       $847.8566   $847.85   $847.85   $847.70