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 $7.70   8.290%   $8.3383   $8.34   $8.35   $8.30  
  Purchase #2 $38.92   4.320%   $40.6013   $40.60   $40.60   $40.60  
  Purchase #3 $194.60   11.363%   $216.7124   $216.71   $216.70   $216.70  
  Purchase #4 $191.41   7.181%   $205.1552   $205.16   $205.15   $205.20  
  Purchase #5 $43.66   11.185%   $48.5434   $48.54   $48.55   $48.50  
  Purchase #6 $14.39   3.598%   $14.9078   $14.91   $14.90   $14.90  
  Purchase #7 $99.80   7.293%   $107.0784   $107.08   $107.10   $107.10  
  7 purchase totals = $590.48       $641.3368   $641.34   $641.35   $641.30