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 $34.49   13.754%   $39.2338   $39.23   $39.25   $39.20  
  Purchase #2 $226.85   4.320%   $236.6499   $236.65   $236.65   $236.60  
  Purchase #3 $196.77   7.472%   $211.4727   $211.47   $211.45   $211.50  
  Purchase #4 $179.67   12.638%   $202.3767   $202.38   $202.40   $202.40  
  Purchase #5 $139.84   3.282%   $144.4295   $144.43   $144.45   $144.40  
  Purchase #6 $123.33   12.618%   $138.8918   $138.89   $138.90   $138.90  
  Purchase #7 $49.49   6.288%   $52.6019   $52.60   $52.60   $52.60  
  7 purchase totals = $950.44       $1,025.6563   $1,025.65   $1,025.70   $1,025.60