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 $169.57   13.123%   $191.8227   $191.82   $191.80   $191.80  
  Purchase #2 $70.29   12.445%   $79.0376   $79.04   $79.05   $79.00  
  Purchase #3 $214.27   3.461%   $221.6859   $221.69   $221.70   $221.70  
  Purchase #4 $57.46   8.854%   $62.5475   $62.55   $62.55   $62.50  
  Purchase #5 $111.79   9.740%   $122.6783   $122.68   $122.70   $122.70  
  Purchase #6 $144.33   12.455%   $162.3063   $162.31   $162.30   $162.30  
  Purchase #7 $59.55   4.525%   $62.2446   $62.24   $62.25   $62.20  
  7 purchase totals = $827.26       $902.3229   $902.33   $902.35   $902.20