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 $192.84   13.777%   $219.4076   $219.41   $219.40   $219.40  
  Purchase #2 $56.45   13.424%   $64.0278   $64.03   $64.05   $64.00  
  Purchase #3 $11.86   8.666%   $12.8878   $12.89   $12.90   $12.90  
  Purchase #4 $141.30   8.415%   $153.1904   $153.19   $153.20   $153.20  
  Purchase #5 $133.33   9.585%   $146.1097   $146.11   $146.10   $146.10  
  Purchase #6 $172.29   10.157%   $189.7895   $189.79   $189.80   $189.80  
  Purchase #7 $179.91   4.358%   $187.7505   $187.75   $187.75   $187.80  
  7 purchase totals = $887.98       $973.1633   $973.17   $973.20   $973.20