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 $128.49   5.296%   $135.2948   $135.29   $135.30   $135.30  
  Purchase #2 $28.51   5.184%   $29.988   $29.99   $30.00   $30.00  
  Purchase #3 $81.36   13.375%   $92.2419   $92.24   $92.25   $92.20  
  Purchase #4 $27.61   3.395%   $28.5474   $28.55   $28.55   $28.50  
  Purchase #5 $108.67   10.726%   $120.3259   $120.33   $120.35   $120.30  
  Purchase #6 $241.59   4.656%   $252.8384   $252.84   $252.85   $252.80  
  Purchase #7 $164.98   10.131%   $181.6941   $181.69   $181.70   $181.70  
  7 purchase totals = $781.21       $840.9305   $840.93   $841.00   $840.80