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 $133.48   3.718%   $138.4428   $138.44   $138.45   $138.40  
  Purchase #2 $46.46   6.401%   $49.4339   $49.43   $49.45   $49.40  
  Purchase #3 $135.96   10.391%   $150.0876   $150.09   $150.10   $150.10  
  Purchase #4 $170.28   10.540%   $188.2275   $188.23   $188.25   $188.20  
  Purchase #5 $93.88   13.212%   $106.2834   $106.28   $106.30   $106.30  
  Purchase #6 $168.39   6.351%   $179.0844   $179.08   $179.10   $179.10  
  Purchase #7 $45.91   13.560%   $52.1354   $52.14   $52.15   $52.10  
  7 purchase totals = $794.36       $863.6950   $863.69   $863.80   $863.60