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 $115.68   13.414%   $131.1973   $131.20   $131.20   $131.20  
  Purchase #2 $191.38   4.430%   $199.8581   $199.86   $199.85   $199.90  
  Purchase #3 $246.94   8.793%   $268.6534   $268.65   $268.65   $268.70  
  Purchase #4 $123.92   7.606%   $133.3454   $133.35   $133.35   $133.30  
  Purchase #5 $79.45   5.102%   $83.5035   $83.50   $83.50   $83.50  
  Purchase #6 $40.35   7.352%   $43.3165   $43.32   $43.30   $43.30  
  Purchase #7 $143.27   11.213%   $159.3349   $159.33   $159.35   $159.30  
  7 purchase totals = $940.99       $1,019.2091   $1,019.21   $1,019.20   $1,019.20