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 $123.43   12.319%   $138.6353   $138.64   $138.65   $138.60  
  Purchase #2 $119.67   6.433%   $127.3684   $127.37   $127.35   $127.40  
  Purchase #3 $150.79   12.452%   $169.5664   $169.57   $169.55   $169.60  
  Purchase #4 $238.61   10.117%   $262.7502   $262.75   $262.75   $262.80  
  Purchase #5 $105.57   9.818%   $115.9349   $115.93   $115.95   $115.90  
  Purchase #6 $22.56   4.232%   $23.5147   $23.51   $23.50   $23.50  
  Purchase #7 $52.51   3.659%   $54.4313   $54.43   $54.45   $54.40  
  7 purchase totals = $813.14       $892.2012   $892.20   $892.20   $892.20