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 $59.78   10.268%   $65.9182   $65.92   $65.90   $65.90  
  Purchase #2 $124.44   7.826%   $134.1787   $134.18   $134.20   $134.20  
  Purchase #3 $113.91   6.296%   $121.0818   $121.08   $121.10   $121.10  
  Purchase #4 $149.85   4.417%   $156.4689   $156.47   $156.45   $156.50  
  Purchase #5 $21.84   13.118%   $24.705   $24.71   $24.70   $24.70  
  Purchase #6 $108.51   11.487%   $120.9745   $120.97   $120.95   $121.00  
  Purchase #7 $63.39   5.698%   $67.002   $67.00   $67.00   $67.00  
  7 purchase totals = $641.72       $690.3291   $690.33   $690.30   $690.40