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 $58.67   12.839%   $66.2026   $66.20   $66.20   $66.20  
  Purchase #2 $16.35   13.643%   $18.5806   $18.58   $18.60   $18.60  
  Purchase #3 $209.90   13.274%   $237.7621   $237.76   $237.75   $237.80  
  Purchase #4 $224.60   11.698%   $250.8737   $250.87   $250.85   $250.90  
  Purchase #5 $127.96   10.421%   $141.2947   $141.29   $141.30   $141.30  
  Purchase #6 $33.55   13.776%   $38.1718   $38.17   $38.15   $38.20  
  Purchase #7 $7.31   9.585%   $8.0107   $8.01   $8.00   $8.00  
  7 purchase totals = $678.34       $760.8962   $760.88   $760.85   $761.00