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 $121.47   7.197%   $130.2122   $130.21   $130.20   $130.20  
  Purchase #2 $75.34   5.776%   $79.6916   $79.69   $79.70   $79.70  
  Purchase #3 $142.65   11.305%   $158.7766   $158.78   $158.80   $158.80  
  Purchase #4 $145.60   12.627%   $163.9849   $163.98   $164.00   $164.00  
  Purchase #5 $189.53   12.582%   $213.3767   $213.38   $213.40   $213.40  
  Purchase #6 $156.74   6.825%   $167.4375   $167.44   $167.45   $167.40  
  Purchase #7 $29.88   8.164%   $32.3194   $32.32   $32.30   $32.30  
  7 purchase totals = $861.21       $945.7989   $945.80   $945.85   $945.80