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 $21.25   8.816%   $23.1234   $23.12   $23.10   $23.10  
  Purchase #2 $70.61   13.551%   $80.1784   $80.18   $80.20   $80.20  
  Purchase #3 $17.65   10.782%   $19.553   $19.55   $19.55   $19.60  
  Purchase #4 $72.32   12.774%   $81.5582   $81.56   $81.55   $81.60  
  Purchase #5 $121.29   6.420%   $129.0768   $129.08   $129.10   $129.10  
  Purchase #6 $109.73   3.351%   $113.4071   $113.41   $113.40   $113.40  
  Purchase #7 $79.35   9.865%   $87.1779   $87.18   $87.20   $87.20  
  7 purchase totals = $492.20       $534.0748   $534.08   $534.10   $534.20