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 $18.34   9.683%   $20.1159   $20.12   $20.10   $20.10  
  Purchase #2 $21.50   3.146%   $22.1764   $22.18   $22.20   $22.20  
  Purchase #3 $127.75   5.196%   $134.3879   $134.39   $134.40   $134.40  
  Purchase #4 $242.41   11.513%   $270.3187   $270.32   $270.30   $270.30  
  Purchase #5 $138.44   12.283%   $155.4446   $155.44   $155.45   $155.40  
  Purchase #6 $103.96   13.663%   $118.1641   $118.16   $118.15   $118.20  
  Purchase #7 $106.47   6.319%   $113.1978   $113.20   $113.20   $113.20  
  7 purchase totals = $758.87       $833.8054   $833.81   $833.80   $833.80