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 $101.85   6.830%   $108.8064   $108.81   $108.80   $108.80  
  Purchase #2 $74.96   12.272%   $84.1591   $84.16   $84.15   $84.20  
  Purchase #3 $94.93   10.110%   $104.5274   $104.53   $104.55   $104.50  
  Purchase #4 $117.93   6.415%   $125.4952   $125.50   $125.50   $125.50  
  Purchase #5 $119.26   3.185%   $123.0584   $123.06   $123.05   $123.10  
  Purchase #6 $12.60   9.734%   $13.8265   $13.83   $13.85   $13.80  
  Purchase #7 $75.70   5.236%   $79.6637   $79.66   $79.65   $79.70  
  7 purchase totals = $597.23       $639.5367   $639.55   $639.55   $639.60