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 $83.36   8.459%   $90.4114   $90.41   $90.40   $90.40  
  Purchase #2 $11.67   6.610%   $12.4414   $12.44   $12.45   $12.40  
  Purchase #3 $110.58   10.589%   $122.2893   $122.29   $122.30   $122.30  
  Purchase #4 $195.70   10.666%   $216.5734   $216.57   $216.55   $216.60  
  Purchase #5 $19.64   3.568%   $20.3408   $20.34   $20.35   $20.30  
  Purchase #6 $153.83   7.313%   $165.0796   $165.08   $165.10   $165.10  
  Purchase #7 $88.75   4.782%   $92.994   $92.99   $93.00   $93.00  
  7 purchase totals = $663.53       $720.1299   $720.12   $720.15   $720.10