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 $71.95   11.661%   $80.3401   $80.34   $80.35   $80.30  
  Purchase #2 $179.66   13.168%   $203.3176   $203.32   $203.30   $203.30  
  Purchase #3 $226.95   5.424%   $239.2598   $239.26   $239.25   $239.30  
  Purchase #4 $133.97   6.612%   $142.8281   $142.83   $142.85   $142.80  
  Purchase #5 $192.85   13.559%   $218.9985   $219.00   $219.00   $219.00  
  Purchase #6 $52.37   4.869%   $54.9199   $54.92   $54.90   $54.90  
  Purchase #7 $74.59   12.190%   $83.6825   $83.68   $83.70   $83.70  
  7 purchase totals = $932.34       $1,023.3465   $1,023.35   $1,023.35   $1,023.30