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 $27.38   5.406%   $28.8602   $28.86   $28.85   $28.90  
  Purchase #2 $225.70   6.350%   $240.032   $240.03   $240.05   $240.00  
  Purchase #3 $15.27   13.601%   $17.3469   $17.35   $17.35   $17.30  
  Purchase #4 $22.92   9.351%   $25.0632   $25.06   $25.05   $25.10  
  Purchase #5 $25.69   7.232%   $27.5479   $27.55   $27.55   $27.50  
  Purchase #6 $112.94   9.112%   $123.2311   $123.23   $123.25   $123.20  
  Purchase #7 $202.70   7.530%   $217.9633   $217.96   $217.95   $218.00  
  7 purchase totals = $632.60       $680.0446   $680.04   $680.05   $680.00