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 $215.96   12.516%   $242.9896   $242.99   $243.00   $243.00  
  Purchase #2 $114.40   8.327%   $123.9261   $123.93   $123.95   $123.90  
  Purchase #3 $36.63   3.221%   $37.8099   $37.81   $37.80   $37.80  
  Purchase #4 $226.55   4.588%   $236.9441   $236.94   $236.95   $236.90  
  Purchase #5 $99.45   9.435%   $108.8331   $108.83   $108.85   $108.80  
  Purchase #6 $176.50   6.138%   $187.3336   $187.33   $187.35   $187.30  
  Purchase #7 $23.80   6.317%   $25.3034   $25.30   $25.30   $25.30  
  7 purchase totals = $893.29       $963.1398   $963.13   $963.20   $963.00