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 $65.93   6.211%   $70.0249   $70.02   $70.00   $70.00  
  Purchase #2 $144.61   11.568%   $161.3385   $161.34   $161.35   $161.30  
  Purchase #3 $113.37   6.190%   $120.3876   $120.39   $120.40   $120.40  
  Purchase #4 $149.93   10.293%   $165.3623   $165.36   $165.35   $165.40  
  Purchase #5 $192.75   8.498%   $209.1299   $209.13   $209.15   $209.10  
  Purchase #6 $81.39   6.367%   $86.5721   $86.57   $86.55   $86.60  
  Purchase #7 $145.58   8.674%   $158.2076   $158.21   $158.20   $158.20  
  7 purchase totals = $893.56       $971.0229   $971.02   $971.00   $971.00