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 $9.73   7.339%   $10.4441   $10.44   $10.45   $10.40  
  Purchase #2 $134.46   4.463%   $140.4609   $140.46   $140.45   $140.50  
  Purchase #3 $90.40   10.637%   $100.0158   $100.02   $100.00   $100.00  
  Purchase #4 $164.74   6.736%   $175.8369   $175.84   $175.85   $175.80  
  Purchase #5 $31.35   8.867%   $34.1298   $34.13   $34.15   $34.10  
  Purchase #6 $197.52   7.559%   $212.4505   $212.45   $212.45   $212.50  
  Purchase #7 $17.56   4.407%   $18.3339   $18.33   $18.35   $18.30  
  7 purchase totals = $645.76       $691.6719   $691.67   $691.70   $691.60