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 $125.57   9.752%   $137.8156   $137.82   $137.80   $137.80  
  Purchase #2 $98.98   7.623%   $106.5252   $106.53   $106.55   $106.50  
  Purchase #3 $11.95   13.452%   $13.5575   $13.56   $13.55   $13.60  
  Purchase #4 $132.94   3.261%   $137.2752   $137.28   $137.30   $137.30  
  Purchase #5 $126.28   9.179%   $137.8712   $137.87   $137.85   $137.90  
  Purchase #6 $168.47   12.173%   $188.9779   $188.98   $189.00   $189.00  
  Purchase #7 $57.55   5.871%   $60.9288   $60.93   $60.95   $60.90  
  7 purchase totals = $721.74       $782.9514   $782.97   $783.00   $783.00