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 $21.38   7.198%   $22.9189   $22.92   $22.90   $22.90  
  Purchase #2 $107.67   9.639%   $118.0483   $118.05   $118.05   $118.00  
  Purchase #3 $110.95   8.504%   $120.3852   $120.39   $120.40   $120.40  
  Purchase #4 $11.34   8.296%   $12.2808   $12.28   $12.30   $12.30  
  Purchase #5 $117.79   5.580%   $124.3627   $124.36   $124.35   $124.40  
  Purchase #6 $209.48   8.222%   $226.7034   $226.70   $226.70   $226.70  
  Purchase #7 $12.69   5.477%   $13.385   $13.39   $13.40   $13.40  
  7 purchase totals = $591.30       $638.0843   $638.09   $638.10   $638.10