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 $231.74   8.624%   $251.7253   $251.73   $251.75   $251.70  
  Purchase #2 $31.93   8.580%   $34.6696   $34.67   $34.65   $34.70  
  Purchase #3 $197.92   9.551%   $216.8233   $216.82   $216.80   $216.80  
  Purchase #4 $205.35   6.119%   $217.9154   $217.92   $217.90   $217.90  
  Purchase #5 $40.45   7.782%   $43.5978   $43.60   $43.60   $43.60  
  Purchase #6 $212.90   6.613%   $226.9791   $226.98   $227.00   $227.00  
  Purchase #7 $10.63   10.868%   $11.7853   $11.79   $11.80   $11.80  
  7 purchase totals = $930.92       $1,003.4958   $1,003.51   $1,003.50   $1,003.50