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 $18.92   5.704%   $19.9992   $20.00   $20.00   $20.00  
  Purchase #2 $143.78   12.362%   $161.5541   $161.55   $161.55   $161.60  
  Purchase #3 $83.58   6.611%   $89.1055   $89.11   $89.10   $89.10  
  Purchase #4 $165.71   5.414%   $174.6815   $174.68   $174.70   $174.70  
  Purchase #5 $90.82   3.737%   $94.2139   $94.21   $94.20   $94.20  
  Purchase #6 $183.85   7.198%   $197.0835   $197.08   $197.10   $197.10  
  Purchase #7 $189.88   11.299%   $211.3345   $211.33   $211.35   $211.30  
  7 purchase totals = $876.54       $947.9722   $947.96   $948.00   $948.00