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 $29.25   8.647%   $31.7792   $31.78   $31.80   $31.80  
  Purchase #2 $45.74   10.777%   $50.6694   $50.67   $50.65   $50.70  
  Purchase #3 $169.58   6.845%   $181.1878   $181.19   $181.20   $181.20  
  Purchase #4 $150.27   8.162%   $162.535   $162.54   $162.55   $162.50  
  Purchase #5 $161.40   9.173%   $176.2052   $176.21   $176.20   $176.20  
  Purchase #6 $25.33   7.835%   $27.3146   $27.31   $27.30   $27.30  
  Purchase #7 $7.91   9.857%   $8.6897   $8.69   $8.70   $8.70  
  7 purchase totals = $589.48       $638.3809   $638.39   $638.40   $638.40