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 $6.99   6.191%   $7.4228   $7.42   $7.40   $7.40  
  Purchase #2 $106.91   10.525%   $118.1623   $118.16   $118.15   $118.20  
  Purchase #3 $155.70   7.768%   $167.7948   $167.79   $167.80   $167.80  
  Purchase #4 $220.93   13.633%   $251.0494   $251.05   $251.05   $251.00  
  Purchase #5 $61.73   10.300%   $68.0882   $68.09   $68.10   $68.10  
  Purchase #6 $56.65   3.240%   $58.4855   $58.49   $58.50   $58.50  
  Purchase #7 $226.37   4.293%   $236.0881   $236.09   $236.10   $236.10  
  7 purchase totals = $835.28       $907.0911   $907.09   $907.10   $907.10