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 $233.97   10.702%   $259.0095   $259.01   $259.00   $259.00  
  Purchase #2 $175.83   11.400%   $195.8746   $195.87   $195.85   $195.90  
  Purchase #3 $145.47   4.618%   $152.1878   $152.19   $152.20   $152.20  
  Purchase #4 $14.36   13.779%   $16.3387   $16.34   $16.35   $16.30  
  Purchase #5 $123.87   11.494%   $138.1076   $138.11   $138.10   $138.10  
  Purchase #6 $150.69   8.208%   $163.0586   $163.06   $163.05   $163.10  
  Purchase #7 $62.82   7.210%   $67.3493   $67.35   $67.35   $67.30  
  7 purchase totals = $907.01       $991.9261   $991.93   $991.90   $991.90