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 $6.55   12.595%   $7.375   $7.38   $7.40   $7.40  
  Purchase #2 $5.64   9.798%   $6.1926   $6.19   $6.20   $6.20  
  Purchase #3 $116.51   7.717%   $125.5011   $125.50   $125.50   $125.50  
  Purchase #4 $118.33   12.506%   $133.1283   $133.13   $133.15   $133.10  
  Purchase #5 $147.88   5.396%   $155.8596   $155.86   $155.85   $155.90  
  Purchase #6 $69.25   7.165%   $74.2118   $74.21   $74.20   $74.20  
  Purchase #7 $10.99   3.576%   $11.383   $11.38   $11.40   $11.40  
  7 purchase totals = $475.15       $513.6514   $513.65   $513.70   $513.70