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 $9.75   6.590%   $10.3925   $10.39   $10.40   $10.40  
  Purchase #2 $34.72   13.327%   $39.3471   $39.35   $39.35   $39.30  
  Purchase #3 $74.59   9.721%   $81.8409   $81.84   $81.85   $81.80  
  Purchase #4 $15.31   6.658%   $16.3293   $16.33   $16.35   $16.30  
  Purchase #5 $24.70   12.675%   $27.8307   $27.83   $27.85   $27.80  
  Purchase #6 $158.32   6.135%   $168.0329   $168.03   $168.05   $168.00  
  Purchase #7 $192.37   3.835%   $199.7474   $199.75   $199.75   $199.70  
  7 purchase totals = $509.76       $543.5208   $543.52   $543.60   $543.30