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 $226.51   12.701%   $255.279   $255.28   $255.30   $255.30  
  Purchase #2 $7.76   3.490%   $8.0308   $8.03   $8.05   $8.00  
  Purchase #3 $49.72   3.601%   $51.5104   $51.51   $51.50   $51.50  
  Purchase #4 $161.42   7.732%   $173.901   $173.90   $173.90   $173.90  
  Purchase #5 $19.66   4.865%   $20.6165   $20.62   $20.60   $20.60  
  Purchase #6 $165.92   9.661%   $181.9495   $181.95   $181.95   $181.90  
  Purchase #7 $244.85   9.395%   $267.8537   $267.85   $267.85   $267.90  
  7 purchase totals = $875.84       $959.1409   $959.14   $959.15   $959.10