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 $27.35   8.452%   $29.6616   $29.66   $29.65   $29.70  
  Purchase #2 $203.73   3.248%   $210.3472   $210.35   $210.35   $210.30  
  Purchase #3 $240.92   3.707%   $249.8509   $249.85   $249.85   $249.90  
  Purchase #4 $152.59   7.866%   $164.5927   $164.59   $164.60   $164.60  
  Purchase #5 $241.29   9.675%   $264.6348   $264.63   $264.65   $264.60  
  Purchase #6 $247.81   10.276%   $273.275   $273.28   $273.30   $273.30  
  Purchase #7 $162.79   11.658%   $181.7681   $181.77   $181.75   $181.80  
  7 purchase totals = $1,276.48       $1,374.1303   $1,374.13   $1,374.15   $1,374.20