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 $237.39   12.611%   $267.3273   $267.33   $267.35   $267.30  
  Purchase #2 $161.92   10.297%   $178.5929   $178.59   $178.60   $178.60  
  Purchase #3 $7.26   12.805%   $8.1896   $8.19   $8.20   $8.20  
  Purchase #4 $238.53   11.714%   $266.4714   $266.47   $266.45   $266.50  
  Purchase #5 $13.60   12.649%   $15.3203   $15.32   $15.30   $15.30  
  Purchase #6 $181.62   10.784%   $201.2059   $201.21   $201.20   $201.20  
  Purchase #7 $58.98   3.280%   $60.9145   $60.91   $60.90   $60.90  
  7 purchase totals = $899.30       $998.0219   $998.02   $998.00   $998.00