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 $36.88   8.145%   $39.8839   $39.88   $39.90   $39.90  
  Purchase #2 $38.64   9.661%   $42.373   $42.37   $42.35   $42.40  
  Purchase #3 $223.58   13.788%   $254.4072   $254.41   $254.40   $254.40  
  Purchase #4 $120.62   12.689%   $135.9255   $135.93   $135.95   $135.90  
  Purchase #5 $143.47   6.547%   $152.863   $152.86   $152.85   $152.90  
  Purchase #6 $120.67   13.153%   $136.5417   $136.54   $136.55   $136.50  
  Purchase #7 $228.29   7.642%   $245.7359   $245.74   $245.75   $245.70  
  7 purchase totals = $912.15       $1,007.7302   $1,007.73   $1,007.75   $1,007.70