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 $80.85   12.639%   $91.0686   $91.07   $91.05   $91.10  
  Purchase #2 $25.70   6.775%   $27.4412   $27.44   $27.45   $27.40  
  Purchase #3 $172.41   7.534%   $185.3994   $185.40   $185.40   $185.40  
  Purchase #4 $103.73   4.385%   $108.2786   $108.28   $108.30   $108.30  
  Purchase #5 $197.40   10.500%   $218.127   $218.13   $218.15   $218.10  
  Purchase #6 $118.37   8.252%   $128.1379   $128.14   $128.15   $128.10  
  Purchase #7 $204.44   13.107%   $231.236   $231.24   $231.25   $231.20  
  7 purchase totals = $902.90       $989.6887   $989.70   $989.75   $989.60