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 $8.45   11.472%   $9.4194   $9.42   $9.40   $9.40  
  Purchase #2 $86.53   9.186%   $94.4786   $94.48   $94.50   $94.50  
  Purchase #3 $241.25   13.666%   $274.2192   $274.22   $274.20   $274.20  
  Purchase #4 $63.57   8.304%   $68.8489   $68.85   $68.85   $68.80  
  Purchase #5 $135.59   3.620%   $140.4984   $140.50   $140.50   $140.50  
  Purchase #6 $143.79   7.320%   $154.3154   $154.32   $154.30   $154.30  
  Purchase #7 $126.32   10.470%   $139.5457   $139.55   $139.55   $139.50  
  7 purchase totals = $805.50       $881.3256   $881.34   $881.30   $881.20