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 $29.38   12.173%   $32.9564   $32.96   $32.95   $33.00  
  Purchase #2 $250.53   13.476%   $284.2914   $284.29   $284.30   $284.30  
  Purchase #3 $23.73   12.385%   $26.669   $26.67   $26.65   $26.70  
  Purchase #4 $204.48   6.814%   $218.4133   $218.41   $218.40   $218.40  
  Purchase #5 $213.60   7.770%   $230.1967   $230.20   $230.20   $230.20  
  Purchase #6 $36.28   9.209%   $39.621   $39.62   $39.60   $39.60  
  Purchase #7 $67.52   5.439%   $71.1924   $71.19   $71.20   $71.20  
  7 purchase totals = $825.52       $903.3402   $903.34   $903.30   $903.40