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 $49.63   12.818%   $55.9916   $55.99   $56.00   $56.00  
  Purchase #2 $242.94   5.253%   $255.7016   $255.70   $255.70   $255.70  
  Purchase #3 $196.87   7.231%   $211.1057   $211.11   $211.10   $211.10  
  Purchase #4 $220.47   4.671%   $230.7682   $230.77   $230.75   $230.80  
  Purchase #5 $245.59   11.729%   $274.3953   $274.40   $274.40   $274.40  
  Purchase #6 $60.91   13.123%   $68.9032   $68.90   $68.90   $68.90  
  Purchase #7 $125.94   9.579%   $138.0038   $138.00   $138.00   $138.00  
  7 purchase totals = $1,142.35       $1,234.8694   $1,234.87   $1,234.85   $1,234.90