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 $204.45   8.374%   $221.5706   $221.57   $221.55   $221.60  
  Purchase #2 $121.67   11.253%   $135.3615   $135.36   $135.35   $135.40  
  Purchase #3 $245.27   7.684%   $264.1165   $264.12   $264.10   $264.10  
  Purchase #4 $78.73   10.171%   $86.7376   $86.74   $86.75   $86.70  
  Purchase #5 $80.89   5.627%   $85.4417   $85.44   $85.45   $85.40  
  Purchase #6 $179.41   12.547%   $201.9206   $201.92   $201.90   $201.90  
  Purchase #7 $34.32   3.871%   $35.6485   $35.65   $35.65   $35.60  
  7 purchase totals = $944.74       $1,030.7970   $1,030.80   $1,030.75   $1,030.70