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 $90.34   12.167%   $101.3317   $101.33   $101.35   $101.30  
  Purchase #2 $103.64   12.760%   $116.8645   $116.86   $116.85   $116.90  
  Purchase #3 $148.42   11.208%   $165.0549   $165.05   $165.05   $165.10  
  Purchase #4 $148.70   12.398%   $167.1358   $167.14   $167.15   $167.10  
  Purchase #5 $100.39   4.834%   $105.2429   $105.24   $105.25   $105.20  
  Purchase #6 $5.97   8.739%   $6.4917   $6.49   $6.50   $6.50  
  Purchase #7 $215.86   5.745%   $228.2612   $228.26   $228.25   $228.30  
  7 purchase totals = $813.32       $890.3827   $890.37   $890.40   $890.40