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 $250.75   12.461%   $281.996   $282.00   $282.00   $282.00  
  Purchase #2 $6.70   6.496%   $7.1352   $7.14   $7.15   $7.10  
  Purchase #3 $24.51   6.478%   $26.0978   $26.10   $26.10   $26.10  
  Purchase #4 $100.94   3.615%   $104.589   $104.59   $104.60   $104.60  
  Purchase #5 $49.53   12.581%   $55.7614   $55.76   $55.75   $55.80  
  Purchase #6 $93.98   3.442%   $97.2148   $97.21   $97.20   $97.20  
  Purchase #7 $223.65   13.531%   $253.9121   $253.91   $253.90   $253.90  
  7 purchase totals = $750.06       $826.7063   $826.71   $826.70   $826.70