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 $28.45   10.602%   $31.4663   $31.47   $31.45   $31.50  
  Purchase #2 $36.32   8.401%   $39.3712   $39.37   $39.35   $39.40  
  Purchase #3 $21.94   8.388%   $23.7803   $23.78   $23.80   $23.80  
  Purchase #4 $42.44   12.870%   $47.902   $47.90   $47.90   $47.90  
  Purchase #5 $139.56   13.267%   $158.0754   $158.08   $158.10   $158.10  
  Purchase #6 $155.88   7.498%   $167.5679   $167.57   $167.55   $167.60  
  Purchase #7 $36.34   10.182%   $40.0401   $40.04   $40.05   $40.00  
  7 purchase totals = $460.93       $508.2032   $508.21   $508.20   $508.30