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 $111.70   9.346%   $122.1395   $122.14   $122.15   $122.10  
  Purchase #2 $165.48   8.137%   $178.9451   $178.95   $178.95   $178.90  
  Purchase #3 $230.50   7.586%   $247.9857   $247.99   $248.00   $248.00  
  Purchase #4 $202.37   12.643%   $227.9556   $227.96   $227.95   $228.00  
  Purchase #5 $162.27   13.623%   $184.376   $184.38   $184.40   $184.40  
  Purchase #6 $8.31   12.265%   $9.3292   $9.33   $9.35   $9.30  
  Purchase #7 $46.98   7.572%   $50.5373   $50.54   $50.55   $50.50  
  7 purchase totals = $927.61       $1,021.2684   $1,021.29   $1,021.35   $1,021.20