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 $144.85   10.459%   $159.9999   $160.00   $160.00   $160.00  
  Purchase #2 $73.55   11.592%   $82.0759   $82.08   $82.10   $82.10  
  Purchase #3 $180.76   10.133%   $199.0764   $199.08   $199.10   $199.10  
  Purchase #4 $119.71   12.518%   $134.6953   $134.70   $134.70   $134.70  
  Purchase #5 $152.51   11.220%   $169.6216   $169.62   $169.60   $169.60  
  Purchase #6 $84.51   3.853%   $87.7662   $87.77   $87.75   $87.80  
  Purchase #7 $146.52   7.549%   $157.5808   $157.58   $157.60   $157.60  
  7 purchase totals = $902.41       $990.8161   $990.83   $990.85   $990.90